A good water pump should cost around $65, just stay away from those cheap POS that discount sells. Might want to try Central Autoparts up on 17-92 right after 434, thats where I use to get my parts.
Replacing a water pump is a fairly easy job. Shouldn't take you more than 3 hours tops and thats if you are slowly doing the work. Just watch out for the water pump bolts themselves. I broke two of mine when I was replacing my water pump in the convertible, one really sucked because it broke off in the block.
I would personally just make sure that its a bad water pump first, they usually weap out of the little hole on the bottom if they are bad. If you could pressure test the coolant system you would easily be able to determine whats at fault. I say look over the two little hoses on the right side of the water pump first. I have seen many cases where these get swelled up and start to leak.