Over on Swappa an unlocked 128GB iPhone 6 has an average selling price of around $470, with the last one sold fetching $430. Based on this small amount of information, I'd say a fair market price to ask would be between $440 and $500. It would really depend on how quickly you want to sell it. The prices are not as high for carrier locked versions.
It also seems that the 64GB versions generally sell for $50 more than the 16GB versions, while the 128GB versions are only about $25 more than that.
Your cases, u less they're premium stuff, aren't likely to add much value. I've been sitting on some 5s cases for a whole that I just don't want to give away (we are getting at least one SE so that turned out well, I could say haha).
I would try and ask between $450-500 but it also depends where you're selling.