The JBJ regulator has all you need (bubble counter, check valve and solenoid) and it usually runs around $80. It is much better than the Milwaukee regulator in that it has a built-in check valve which can be quite expensive.
As for the tank, you can get a 10lb tank used or brand new (I can't really remember) on eBay for $25.
Depending on what your going with; the best diffuser (by Eheim) costs $15, while Marc at Aquatic Store has a good DIY reactor for $20.
So the total cost w/tank is around $120-125. Good luck!