Well the Cosworth's are going to be more money, they've actually become quite collectable over the last 8-10 years. Very unique car. May as well stay away from those if you want cheap.
Even the 71-73 vegas are going to be more expensive than 74 and up. Simply because the 71-73 are the more desirable body style, after that most people consider them ugly. Any Vega worth it's own weight (meaning a good solid foundation to work with) is going to bring some coin now. Very popular bracket car for some people, and the ones that weren't taken off the street were run into the ground, everything East of the Mississippi rusted to a pile of dust.
I've had about 8-10 of these things over the years. 20 years ago they were cheap to buy, so I'd come out here, buy a rust free one, take it back East and use it as my daily driver until the salt ate it to death. I'd pull the drivetrain, come back out here and buy another, take it back East and drive it until the salt ate it away,,,,,see a pattern here?
I had collected them for quite some time and even had a few left before I moved out here. I even had a few Monza Spyders which is the same H-body platform that came out in 1975.
Finding one now isn't hard,,,,finding one cheap is.....you would be better off finding one mostly complete and pay more for it, and then hope the interior is in good shape, because nothing is reproduced. That's where looking for one out here is going to bite you. Might be rust free but the interior will likely be scorched. Putting a basket case together will likely cost more than paying for a decent roller that is already there.
I used to have extra seats that were very nice, extra consoles, a couple of dash pads and more door panels than I could count. Good stuff to have. So unless you plan to go completely custom and hand made, it will pay to find one with nice interior intact.
Look around enough, and if you are willing to travel, you'll eventually find what you are looking for.