Unfortunately Swappa averages are dropping literally daily as more and more listed for dirt cheap. Right now a mint 64gb are as low as $430. After $10 Swappa fee and paypal you're not even netting $100 for OP's situation.
You'll see a lot listed a hair over $500 but they aren't selling. I had my mint 64gb unlocked up thre for $508 for 40 days and didnt get a bite. They aren't selling for those asking prices at the end of the day if you look at the past sales section.
The avg price dropped $120 from August to September from $540 to $420.
Paying in $300 just to risk making a profit is a risk no matter how you look at it. If it was already paid off it's no loss.
That's the problem, people expect to sell their phones for too much. Or they don't know when to buy and sell, timing is everything in a constantly depreciating market.
I sold my 6 (64gb) on Swappa the day of the Apple keynote for $489 (underpriced for good condition). And I purchased a 6 plus (64gb) the weekend after the new iPhone release for $500 (mint condition). 6 Pluses are now going for much lower than that, but it was still a good time to buy.
Too many people price their phones inaccurately at the wrong time and they never sell. All the iPhone 6 models being sold for over $500 on Swappa right now are laughable. Good luck. My iPhone 6 sold within 7 hours. When I sold my iPhone 5 before that to buy the 6, I priced it $30 bucks lower than the rest of the phones and it sold within 2 hours. Most people aren't willing to make their phones somewhat of a deal. I would rather sell it now for a little less over selling it later for a lower market rate (or possibly not at all). And yep, there are some suckers who will snatch that high priced phone without a second thought, but I don't waste the time chancing it.
I do understand a good number of people can't be without a premium phone for a couple of weeks. Or don't have the patience to maximize their profit. I have a cheap Android LG something that I purchased many years back that I just swap my SIM into for the 2 weeks I'm without a phone (you can buy them on eBay for like 40 bucks). But that method isn't for everyone.
As far as the OP goes, I agree he most likely won't be netting much profit from selling a leased phone at this time. My comment was in regards to Swappa being a much easier (and cheaper) outlet to sell phones than eBay is. That's all.