They say you're supposed to spend about 2 months salary on an engagement ring, so I think financing a ring is a lot more common than it seems... But since that's not an option, and you're looking for a quickie wedding, there might be a compromise.

Does your mom/his mom/your grandmothers/his grandmothers still wear their engagement ring? If not, ask if you can have the diamond. Removing the diamond from the price of the ring (so you pay for a band, a claw, and whatever sidestones you may want) can significantly reduce it. Possibility?

If shane co won't do that (i've never bought from them) you could go to a real jeweler. They'd be happy to sell you just a setting.