I think a lot of people are over thinking this... Simplicity is always a great go-to. Scott Bradford mentioned above, consider riding off a neighbors wifi for rent credit, it's is a great, easy idea. I'm thinking cheaper is better since this is so temporary.
The cheap option:
Since it's an apartment complex, I'd imagine there are lots of wireless networks in the air. Since your ISP is Comcast, you're in luck. Comcast is trying to create wireless access everywhere. So they came up with a tedious plan that most people are unaware of, where they are using customer modems to broadcast their own wireless network.
If a Comcast customer has a wireless modem, you'll have your normal SSID, probably named something like John Doe Family Wireless, but you'll also see an SSID called xfinitywifi, which broadcasts from the same modem. Xfinitywifi can be connected to, and as long as you have a Comcast account, you can use the network, you just sign in with your account. I'd say if you can see xfinitywifi in the air, login with your Comcast account and use the wifi. I'm sure you can find someone that can give you an account, if you yourself don't have one. Luckily, Comcast has a monopoly in the areas that they serve, so it will be difficult NOT to find an account to use, or even the xfinitywifi network in general.
The not so expensive option:
Do a hack job since it's very very temporary situation. Run an Ethernet cable to the unit if possible. It doesn't have to be pretty, just make sure nobody can cut the wire. You can plug in a wireless router that you have laying around and run the cameras off wireless, or even plug in directly. Then you can have your DVR in a safe distance away from the unit so it doesn't also get stolen.
The cheap but a little more expensive option:
Other people have stated this, but I will reiterate. Setup an access point, and bridge it to an existing wireless network. This will take a little time to configure. I don't know how network savvy you are, but if you can figure out how to setup the AP, then you can do this option. You're looking at the cost of an AP, as well as the time to set up, which could take as little as 20 minutes if you have the know how. Undisclosed Integrator #1 recommended a Ubiquiti bridge. I second that statement. Ubiquiti products are easy to setup and are fairly inexpensive.