Those aren't always terribly expensive if you keep an eye on eBay. I've bought two and haven't paid more than $30 for a marked one (with the seam allowances). They're much less expensive if you're fine with a plain one.
Just keep a close eye on the listings.
Also, you can get stickers (from SewClassic, I think) that go onto the plain ones and show the seam allowances. I think it's something like 5 stickers for $2. I like mine. If it starts to snag the fabric (mine haven't) you can just change the sticker.
Good luck,
Ila