Do you change your own oil? If so, QuickOilDrainValve.com - the best way to change oil
Do you want to hear more wooshing from your intake? If so, do the Snorkus Delete Mod = $price of time/saw/glue if you do it the cheap way where you cut the snorkus and glue a piece over the hole where the silencer used to be or ~$50 if you buy the pre-made GP Moto elbow
Do you want a little more performance inexpensively? If so, K&N drop-in filter = ~$50
A set of 4 braided stainless steel brake hoses is about $100, but then you need brake fluid too
An armrest extension is about $100 and very useful if you need more cargo space.
Trunk cargo nets are very cheap - ~$30 if I remember
Tinted tail light covers - Lamin-x Protective Films - Protect with Style
are around $55 if you're going for that look.
I found Hella Supertones for ~$62 on Truck Accessories, Performance Parts for Pickup Trucks, SUVs, Crossovers, Vans
I got my knock-off STi V-Limited front lip for $105 from eBay, but you might need those extra little pieces on the front sides since you have an 04/05, so that'll probably put you somewhere into the $100-200 price range - OR you could do it the cheap-o way and use a piece of black rubber garage door seal and screws/washers... which looks ok and would probably cost you less than 100
...and then there's always basic maintenance - switching to a good synthetic motor oil or transmission/rear differential fluid... dunno when your next scheduled maintenance is coming up or what type of fluids you've been using, but that's a good way to spend a few extra bucks too, imo.