Late response, but I thought I'd check to see if anyone else has come up with something cheaper. Here's a list I scratched together from co-ar15 and arfcom posts. Wishing I could find some more people with experience at this special occasions place.
Special Occasions (Federal / Belleview - Denver)
$25/receiver including ar-15 under trigger guard
Iron Ridge in Longmont
$$ ??
David Clevinger Engraving
http://www.dcengraving.com/
Approx $100
Forum Engraving
1496 W. Tufts Ave, Englewood
http://www.forumengraving.com/home/home.htm
$50-65
Farmer's Engraving in Cherry Creek, Denver
$50 - (6/8/10 update - just had mine done here and it was only about $20ea, not $50)