I have an 06 GP, I think the best mod's for the 04+ are The SE front air dam/rear valance, SPO spoiler, Rims, Inferno Hood, SPO grille and Hid's.

Cheapest mods I think are SPO grille, Hids and SPO Spoiler.
The SE front air damn and rear valance are only about 250$ each, but you have to include the cost of paint/shipping.

I bought 18" rims/SPO spoiler/SE front air damn for a total of around 700$. It will cost around a total of 1000$ to get 18" tires and those parts painted.