I have a 85 mustang lx 5.0 with a 302 v8 and its a notchback i was told because its a notchback its a rarer mustang is this true??

An 85 LX 5.0 notch is a lot rarer than a GT, no doubt about it.

now if you had a cosworth vega,you would have something.

A Cosworth Vega would be rarer, especially since they have all probably rusted away to the great junkyard in the sky. Like most Vegas. They were a cool car in their day, just like the SVO Mustang. Not too many people understood what they were all about. V-8 engines were the logical choice in their day.