[Erik_in_Hoyt]

first - make sure your ceiling trusses are "bottom load rated" for putting in that ceiling.
second - what kind of price can you get for 4x8 sheets of rigid foil lined foam board? the foil makes a radiant barrier, which increases it's effectiveness and you can tack it up to the studs then install your OSB right over the top of it.
another option would be to put up some visqueen then blow in fiberglass batting. (there's one on the market now with some "tack" to it that sets up in a day or so which limits future settling)

either way, you're going to be spending some $$ - and $1200 doesn't sound too far off considering the size of the building no matter what insulation you go with.