The bottom line cheapest way to go would be a QB78 (the standard version). With careful selection of rings and scope, could come in at about $115-$135. The 78D has a nicer stock and is usually better finished, but it runs on the exact same metal work.
Crosman's 2260 and 1660 SE (the steel breeched versions) are sold with scope and mount, so they cost a bit more...about $175-$180 last time i checked. Could live with those, they are a very simple design that is easy to repair/reseal and replacement parts are cheap.
I've shot the little 2260 with the plastic breech, and if you can NOT SCOPE it, live with the issue iron, it's really not a bad deal. Feels super light, but it shoots well.
All of the crosmans have a kind of sucky trigger, which limits how well you can shoot them (or forces you to spend the $ for a trigger job). Qb's triggers are adjustable, and most people can easily turn a few screws to end up with a nice trigger.
