I'm fighting to admit this on a personal level, but everything I've been told about scope quality is true~you get what you pay for. In the last 12 months, I've spent about $500 on optics. One decent one, one great used one I paid $75 for shipped.
Not that long ago, I'd have said $500=5+ scopes.

I'm no richer than I was then, but the amount of time I have in the field is a fraction of what it once was. When it's finally trigger time, I want some nice glass. As money allows, my optics will likely get upgraded before my rifles do.
Oh yeah, and rifle scopes on air guns~depends on the air gun. If you're not familiar with all the "power plants" in those things, the easiest answer is "no, don't do it" If you know the difference between them, spring-piston guns (sometimes referred to as "break-barrel") are the ones that are the issue..and need AG-rated scopes. Pre-charged pneumatic (PCP), single-stroke/pump-up pneumatics, CO2~anything should work.
Leapers~I've never owned one, but have a "Rex" brand on one of my air rifles and it's excellent. (CO2-powered) Can't say whether or not it's true, but I have read that Rex were made by the company (or are the company) that makes Barska scopes now.