Sure, magicians are rare. That means if you ARE a magician, especially one with security expertise/ability to cast wards, you can write your own meal ticket. It takes a few hours to refresh a ward that's going to last weeks. One contractor can ward dozens of sites all by himself, and that's before figuring the higher security places that can afford to keep an astral mage on retainer.
It lasts for 1 week per net hit on a Sorcery + Magic test against a threshold of 6. What stats are we assuming for a typical wagemage? 10 dice pool? 12? Threshold 6 is a heck of a lot. It also takes four hours to cast. Assuming each ward lasts two weeks (doubtful, given that threshold), one wagemage would max out at 2x per day (don't forget travel time between sites), or 12x per week (even wagemages get one day off), or two dozen maximum wards.
You can throw more mages at the problem, but that isn't going to make it any cheaper. Speaking of cheaper, you also need reagents for at least the temporary lodge you need at the ward site.
Also, at Magic 6, it only encompasses 300 cubic metres - that's a small three storey house. So if you want anythng larger warded, you need more casters and/or people who are Initiated.
Also also, what's an overworked wagemage going to do when a ward gets torn down? If he's halfway through making the next ward somewhere else -- does he stop and abandon it to raise the alarm with the owners of the warded property? Hell, how confusing is it for a wagemage to receive a "oooh a ward was just taken down" spidey-sense if he has two dozen of them up? How does he know which is which?