Keep in mind that in a city the size of Harrisonburg Plan 9 is more than adequate. The selection tends to lean towards alternative and southern rock, as far as the used music is concerned, and whatever rap or hip-hop you'll find tends to be overpriced. But when you consider that Plan 9 is about the only game in town when it comes to used music you can't really fault them on their prices.
Don't get me wrong, when I was in Dayton, OH every neighborhood had a used record store, and while there were chains that sold used records the prices weren't anywhere close to being as expensive as Plan 9's. But if you're talking about how much money you'll get trading in your own records, or about what you can expect to pay for a record, Plan 9 uses the same system to research their records and offers a competitive price. If you really want unusually cheap CDs you should probably go on Ebay or something, particularly if you want more mainstream material. But if alt-rock or the indie stuff is your thing you will probably find it at Plan 9.
Plan 9 also does a lot to promote local artists, something patrons shouldn't take for granted because not all stores do that. Plan 9 was the first independent store I've been in that even has a local section. If you're looking for mainstream stuff, particularly rap, you'll probably pay $10 or more for a used CD. That isn't good when you can get a new CD at Wal Mart for that price, or Circuit City. But for the time being Plan 9 is the place to go for used stuff unless someone is willing to open up another shop.