A bit late I think, but here goes.
There is a low end mummy bag sold in Walmart, fill about one pound, quality seems adequate for summer use.
Occasionally you can find them as low as $30 at the end of summer camping season.
You could use it as a liner and add maybe 25 degrees to the bag you have.
I have two. I bought them largely as liners, but limited use in the summer seems to suggest they are of adequate quality when used alone in friendly conditions.