My suggestion is to not do any power adder like nitrous with an under $500 budget. You cant even get a kit with all of the safety features for that.
That and its probably a good idea to have some extra $$ in case something goes wrong.
Stick to bolt ons. Get a K&N filter, mid-pipe, mufflers, pulleys and insall them yourself.
or
Just buy gears and have them installed, that will tap your whole $500 or close to it.
Just my .02
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