350z magnetic tax disc holder for £5 and stubby aerial for £6 off fleabay, then a decent car shampoo, wax and sponge combo from halfords for the remaining £9.
or, if thats too much for you, heres my list of cheap (free) mods...
wind windows down a touch - better exhaust sound into the cabin
make sure all tyres are at the correct pressure (usually 35psi on 18s) - better handling, better fuel economy
take the spare wheel and jack out the boot - saves 20kgs of weight, giving fractionally better acceleration, braking and handling
ditch the missus - saves xx kilos of weight, better acceleration and handling, plus cures the annoying whine from the nearside seat when pushing on, allowing you to brake later and corner faster