How to play Risk for office fundraiser?
I would veer away from having to paying money to raise armies, which, depending on implementation, might be perceived badly (as either being able to pay money to buy huge armies, or requiring money to even make it to the end by having to pay ever-increasing amounts of money just to play.)

Instead, I would suggest allowing players to pay money to re-roll the dice used for determining battle outcomes. This way, you get a little bit of an advantage from paying money, but you may still roll badly and not win. (plus, it can incentive the other player in the battle to pay money to re-roll his dice, etc.) I might also try a staggered cost - so that for each player, the first 10 re-rolls cost $1, but the next 10 cost $2, etc. 

As for players that have already been defeated, you could allow them to pay for re-rolls on one side in future battles. Depending on how competitive your office is, this may break friendships as people contribute money to their favorite faction, etc.

Also, note that the physical Risk game comes with a limited number of colors, so you might need to find some sort of work-around (have several independent "pods" where the winner of each goes on to play in a final game) or digital version.