04 was the first year of the STi.. Kinda a lame version deffinately worth finding an 05+ if possible.
05 was the same body as the 04 technically but a little better lot of bugs worked out.
06 was a body change went ugly imo also has a different turbo.
07 stayed the same as the 06 but they had a different turbo.
08 is another different body that just got fugly.
2005 the WRX used a 2.0 liter motor
In 2006 the wrx upgraded to a 2.5 liter motor and really good brakes.
2007 wrx stayed pretty much the same as the 06.
2008 wrx body changed and brakes went back to crap.
09 the wrx has a few options... The wrx upgraded the turbo giving 265 hp but stayed with the 5 speed trans and crappy brakes. You can also purchase a WRX GT which is basically the same as the 08 WRX (227 hp)
Prices really vary on mileage, year, location and blah blah best to check out New Cars, Used Cars, Blue Book Values & Car Prices - Kelley Blue Book