Well, I have a lightweight fleece blanket that I use in the summer in the house, the sort you can pick up at Wal-Mart for cheap. I'm going to weigh it shortly. It may not be lighter than a down quilt, but it's not a bad way to save $200 if you don't already have a summer bag.
EDIT: Okay, my lightweight fleece weighs 584g, 20.6oz, and it's 60 x 80". If you trim it to 50 x 70", that's only 15oz. Not as good as I was hoping, but a viable option to save money.
It's not the warmest but if the temps are in the 70s, 80s, what more do you need? Of course, YMMV. Hopefully I get a chance to try out some lightweight blanket options this summer.