My guess is that if you decide to sell, you'd get anywhere from $400-$600 over MSRP.
I'm not doing it, but I've done it before. I was fortunate enough to have the 7+ in Jet Black on launch day.
I had someone offer me $700 over what I paid, so I caved and sold it.
It sucked at the time to wait weeks for the next phone, but now that time has gone by, I don't regret it. It essentially paid for my new phone.
I know people hate scalpers, and I agree for those buying many things (whether it's concert tickets or phones) that skew the market and ruin it for us normal folks, but for those who have one single phone and decide to take a profit, it's hard to fault them.
They're not skewing the market or the experience for the rest of us IMO, and it's a market that Apple purposely creates.
They would never admit it, but they purposely cannot meet demand on day 1. They use the "always leave them wanting more" game better than anyone.