Prime is more expensive ounce per ounce, but you use way less. I used to use tap water conditioner (API) over prime because it was cheaper, but I went through it hundreds of times faster.
AC filters (only good thing hagen does IMO)
Rena Filters (best thing API does, but they have lotsa goodies)
Anything with the name SeaChem on it
And the best money I ever spent on my tank. Reptile can lights filled with daylight CFL's on top of an all glass top to make any smaller tank a high-tech on the cheap.