No sweat, that's what I'm here for.
PT MP9 is still available, but you won't be using your MOTU interface. If you're a student or teacher, $300 for PT9 Academic is a pretty good deal and still available via Amazon and around the internet. Plus it works with your interface and you'll get your upgrade to 10 for free.
If you're not a student, the absolute cheapest option is a MobilePre + MP9. I got mine from Sweetwater for $300. Or get a nicer interface like the new Fast Track - I wish I'd held off 2-3 weeks and gotten a C400+MP9 bundle for my mobile setup, though the MobilePre sounds pretty good. Comes with the iLok and a bunch of free plugins - Eleven LE, Strike, and Velvet.
If you want to go PT10 the cheapest way, bewilderingly, is to buy a PT9 + Mbox Mini setup for around
$600 (I totally dig unique squared, btw, they're just as good as Sweetwater IMO). Mbox Minis are nice, but M-Audio hardware seems to get supported for much longer than Avid/Digi-branded stuff.
Once you figure out the way you want to go, and if you want to go MP, do it and then eBay your MOTU interface for $220-300, depending on the mood of the buyers that day.