Master's institution forcing me to deposit $5000 immediately -- what can I do if I accept? I applied to CS masters programs in the US, and I have recently received an offer from a top-15 institution.

The program is unfunded, and I want to wait for decisions from other programs I applied to, some of which are funded. 

However, the institution is telling me that they are only giving me one week to make my decision (I must respond by 3/13) and that I must deposit a non-refundable $5000 by that date in order to secure my admission. 

As the school is not giving out a funded offer, they need not abide by the April 15th resolution. 

Seems somewhat unethical to me, but in any case, I have some questions:

a) How common is this? For MS programs to force you to deposit huge sums of money within one week of the decision?

b) If I accept this offer, would it somehow notify other schools that I have accepted this offer, and make other schools less likely to give me an offer?

c) Let's say I decide to shell out my 5k -- would it be unethical for me to withdraw at a later date if I get into another school? They're not investing anything into me, and they get free money.

d) If I go the route of c), with whom would I be burning bridges with? Only the admissions folk?
I do not think the quick monetary outlay is common. You seem to be leaning toward acceptance and then backing out if you get a better offer. This may be possible because, in my experience, there is not a centralized system which lets other schools know you have accepted (although perhaps that could be different for Masters programs where you are using federal aid, not sure). 

However, I would encourage you to take an alternative approach. Call them and explain that you are waiting to hear from other programs, some of which would be funded. Ask for a deadline extension until the April 15th deadline. In the past I was successful with this strategy, and I know students in our program have also successfully received extensions.