Some of the more popular mods that are in the "couple-hundred" range..
JWT Popcharger
MotorDyne Plenum Spacer
Most other things are going to cost some dollars. The Z isn't a cheap car to mod...those are probably the 2 cheapest power adders to consider, and the popcharger isn't exactly a big power adder. 350Z's aren't cheap to mod.
Money opens up a lot of doors however...namely turbo/superchargers. Or Nismo parts if you don't want to spend quite that much...like exhaust, header, CAI, etc.