The rear bumper is made out of polyurethane, not steel. Relatively minimal damage can require you to replace it. That doesn't mean the car is totaled. Pics would help us render an opinion.
As for what it's worth, the insurance company doesn't care what you paid for it. And even assuming you paid what it was worth, it's no longer worth that because cars get less valuable by the day.
Insurance companies do, however, tend to try and pay you trade-in value on the car, when you should insist on private-party value. Trade-in can be thought of as wholesale. You won't be able to pay wholesale for a replacement, and therefore you shouldn't have to take wholesale from the ins co.
Did you determine that it has frame damage based on the bumper, or did someone inspect it and tell you it has frame damage?