Just for something to do (stuck watching the kids) I started looking for uppers for sale. Found a few sites that show 20 inch uppers in stock.
Prices for O.K. to really high. Maybe the days of a $500.00 are over.
http://www.cheaperthandirt.com/79131-1.html $720.00
http://grabagun.com/bushmaster-firearms-inter-97102-flttop-rec-450-upper.html $622.13
http://www.hinterlandoutfitters.com/bushmaster-91702-flat-bushmaster-upper-receiver-izzy-suppressor-20in-barrel-p-41628.html $697.85
Some shops are over $800.00. Maybe a call to the place in Mich. that wildcatter recommends would be worth it, if interested.