I use an Exped Synmat 7DLX (extra length and width), review here- http://www.trailspace.com/gear/exped/synmat-7-dlx/
Cost wise mine was �80 from up and under in Cardiff. Packs down very small and is very comfy and warm.
There's a guy who crops up on ebay selling the downmats every now and then cheaply, I'm allergic to down so couldn't get one, but worth a look.