It's not hard to DIY if you take your time. I've done a few, and have taken the top frame off the car before I installed the new top. I think there's a decent write-up on miata.net.
EDIT - Several write-ups here - http://www.miata.net/garage/garagetops.html
If you don't want to tackle it yourself, check with Rogue (Westside) or OZMDD (Eastside) and ask what they charge.