When spec-ing out a main (or any other sail), it's good to compare the grades/construction of New, with Used, and finally with a re-cut.
Also, "new" can range a lot in cost, from composite tech to "mature technology" dacron.
I have successfully had a re-cut done on an old dacron main, which got us by several years until we could afford a new one. Used sails are a lot harder to assess -- everything depends on the honesty of the seller and the age and history of the sail.
Often, to keep the cost down, it is better to buy a new dacron sail with a seasonal discount from a major sail loft chain. Local lofts have lower overhead and can often offer a really good price, too.
Shop around, and take lots of notes.
Don't overlook owners' sites for good deals, like www.ericsonyachts.org.