Ok THIS will take a lot of work, but is reuseable, and with time you will have a portfolio of security to pull up from for future runs.
The first thing to do is to ask yourself these questions:
"Why security?"
Most places can get by with an overly large receptionist, so why does this particular building need more? Is it a top level research facility? It is a data storage office? There must be some reason why they need extra muscle. Just how valuable is it to the parent Corp, and the its division? What is it that actually need protection? Is it a room? a floor? a wing? a box?
"What Security"
All the magically security in the world isn't going to protect a file on a database from a decker. Nor is IC software going to do anything against a physical threat. Once you know what you are guarding, then you can begin to lay the framework for the actual security system!
"How Much Security"
Security is expensive. Guards cost money, Equipment costs money. Drones cost money. Heck, money costs money! How much you spend on security depends on the perceived value of the item being protected. No sense in spending $2 million to protect $50,000 in assets. There is a fine balance between affordable security and idiot security...but we will talk more about that later.... What you are looking for is an annual amount of expenditure for the building. This is will give you an idea of your onsite forces. A fair range for annual expenses is $500,000 to $5,000,000. That might sound like a lot, but if you have 10 guards in your building, there is $300k right there (assuming a low-middle lifestyle). leaving you $200k to operate mag doors, drones, cameras, IC, and whatnot.... But we will discuss this more later.. for now just come up with a dollar value.
"Where security"
Placing is Key, and to do this you need a layout. Nothing fancy, just an property fence line to the outside of the building for outside security. And a outside wall and inside wall sketch for the inside of the building. Make life easy and do only 1 or 2 floor plans, no matter how high your building

They really don't vary a whole lot on the inside (unless your in an "ego piece"). Now that you have an idea of the floor layout, you can see the traffic patterns for your floor.... where the people are moving to and from.... where the entrance and egress off the floor is located..... so now you know your choke points... and this is where security goes.
For security to actually work, it has to be user friendly, unobtrusive, and intuitive. Sadly, a "Fool Proof System" usually breaks all 3 of these and falls apart. let me explain.
User Friendly: The system has to interrupt your routine, to secure your identity, but if it totally throws off and breaks your routine, you cease to be productive, thus the system has crippled you, and thus itself (if there is no production, there is no profit, thus nothing worth protection..).
A computer password that you enter when you log in is very user friendly, but not very secure, as if you forget to log out, you ID is compromised until you get auto logged out, which could be minutes. A Computer password that you have to enter every 15 seconds regardless of activity is incredibly secure, but not very user friendly. Somewhere between these two is a happy balance. Sometimes that balance could be a different approach, like an "eye contact" camera for a computer.... As long as you don't look away from the computer for more then 5 minutes, you stay logged on, thus solving a "time out" issue.
Unobtrusive: There is only a certain amount of hassle anyone will go through for various pay. A security system that causes stress to your employees causes you production, both from new hires being less effective as seasoned workers to the seasoned workers being burned out. A security system that subjected every single person to every single floor to a full body cavity search would be incredibly secure, but I doubt you would have too many people reporting for their 6 day a week job repeatedly. The security system has to be there, It has to be seen (being seen is 90% effective at stopping crime), but it shouldn't hassle the employees any more then it needs to. A circular security door (forcing 1 person at a time) entrance, or a 2 door dead man system, or a turn style, MAD scanners, chemical sniffers are all "unobtrusive" security devices.
Intuitive: Scanners and cameras are all well and good, but the system has to know what it is looking for... So this is a training issue.. is the training and discipline of the site up to snuff? Or is it slacking. Generally, the more quiet, relaxed the building, the more slack the guards. A data storage facility probably has 3 guards on site, the young rookie, the near retiree, and the out of shape middle ager. A crack research and development facility might have a dozen well trained, ex military hard asses who eat, sleep and shit procedure, doctrine, and policy. This is you human element. Does the security guard ignore the pressure alarm on the 10 floor cause "it's always going off"... or does he call in for magical support every time he gets a chill? The problem with an overly automated system is oversight, generally these systems are lacking the required oversight as the guards get tired of staring at monitors showing the same thing for hours, go off and do other things... like play poker. Too Much oversight and not enough automation however, leads to a hassled and rushed, not to mention stress security staff, who are prone to make errors....
*****
So now you have your answers to the questions
You have an idea of your annual budget
You have an idea of the interior and exterior layout.
You have idea of how much and what type of security you need
You have an idea of what you are protecting.
Now the Fun starts

Take your item (whatever it is) and place it in a room (if it IS a room, ignore this step

). This is the epicenter of your security. All your toughest security should be placed near here in a User friendly, Unobtrusive, and intuitive way that is to be expected for this level of security. Now build outward.
Costs are easy. Just use the base cost for your sensors, drones and scanners, remember, you are accounting for repairs and maintenance as well, which means you probably have multiple parts for everything in storage for easy replacement. Over the years it works out. For human guards, look at their professional level and assume $30,000 +$5000/lvl average. Most buildings provide round the clock security... so that means a minimum of 4 guards. And they are usually working alone! (assuming a 50hr work week, 50 weeks a year)
Now save your template for the next time you need a security plan and you have the ground work for the next one... and the next one, and the next one... and soon you will have a portfolio of various specialized security that you can apply on the go to anything to you need.
Once you have all this worked out, you know what loopholes the system that can be exploited and how, can answer character/player questions about the security, and so on.