my first piranha set up ran me somewhere in between $350-$400.
125 gallon tank set up (tank, stand, light, tops, cheap HOB filters) $250
extra filter - used canister filter $50
Heater $25
5 red bellies $35 ($7/fish)
misc
you can also do a single serrasalmus species if you prefer a solo. a small quarter size serra. piranha around here cost between $20-30 each. Prices go up with size and species type.
other than that you can start off with red bellies. They're pretty hardy and easy to take care. Not to mention cheaper.