The Lowfyr Job Johnson just tried to get some runners to kill Lowfyr for like 50K, do you think he wouldn't be prepared for them to say no? Did you think he wasn't EXPECTING them to say no?
I will admit that pulling that stunt in the first place is kind of a 'Bad GM' move. But you do it ONCE... maybe not so extreme... but you do it once to make a point.
As a Johnson, I would be wary of hiring a group of unknowns if they didn't ask questions. If they aren't asking questions, there's a good chance that they have no intention of actually doing the job, just in it for the deposit. I also wouldn't lead with my budget. Now that's for a 'special order' type job. 'Commodity Jobs' are different. Assassinations are a commodity job until you get VIP's. There's a going rate and if the runners don't take the job, you can find someone else to do it. Package delivery, generic mayhem, etc. For the most part, these things don't take much planning or legwork. You'll also find that some people specialize in commodity work. Assassins, smugglers, gangers, etc. They work very much cheaper than a generalist, but they have a narrow focus of what they are good at. The reason VIP assassinations are special order is because VIP's invariably have many layers of differing overlapping defenses. You need a whole team to investigate the defenses and then create the opportunity to even attempt the assassination. That goes back to my assertion that you don't lead with your budget.
Let's say you, as a GM, have a certain idea about how the run could go down. Let's say the Johnson is trying to get a sample of Laes. That could involve a lot of research to find a source, then infiltration into the Tir, breaking into some facility, acquire a sample, then exfiltrate back out to safety. You're looking at a month of planning and operations with bribes and transportation, blah blah blah. A Johnson could expect a run like that to run a hundred thousand nuyen. But when he puts the request on the table, the ex-Ghost elf runner offers up the Laes dart round he was planning on using if this meet went south. 2000 nuyen and the deal is done. If the Johnson led with his budget, that could have been like 20-50K deposit. Admittedly, that's an extreme case... one where the GM didn't look carefully at the character sheets... but in some situations, runners with specialties can do things cheaper than you're expecting. If they are charging more, then you can always present your plan and offer less.
From a Lazy GM perspective you can work it another way. Give them the basics of the run and let them do some planning and figuring. Sit in on it. Guess what... they just wrote the adventure for you. And it is an adventure they want to do. And they even did the math to figure out what is fair. All you have to do is grumble a bit and sign the check.