Oooh, I see, so I guess it is more fun to have goldfish, which are bigger.
I'm actually planning to do this with a 75, too, except with guppies. I'm trying to get some healthy ones (due to bad LFS stock), but I found a good place for feeders now.
If you just want to be the most cost efficient in the long run, I suggest you get some cheap gravel for good bacteria to grow on the bottom and take care of some of the waste. If you do decide to change your mind, guppies would be much more cost efficient.
Here's my plan:
start w/ 200 in a 75 gallon, lose very few due to cramped tank, have a constant supply of feeders.
They'd just keep on breeding, and you'd never have to buy more... Also, when the denitrifying bacteria sets in, you won't have to do many water changes at all.