What will the iPhone 5s cost when the iPhone 6 comes out? How much cheaper will it be?

Question pretty much says it all.

It will most likely decrease $100. If you are signing a new contract it could be free if not it will probably still be at least $500. Both of those prices are for the 16gb model the other ones will be more.

I think It will probably not drop in price much until November and the holiday season.

It will go down between 100-200$

Apple doesn't lower prices until it has to, so don't look for much.

Let me look into my magic crystal ball

When a new iphone is real eases, the previous model is ALWAYS reduced by $100. I would recommend going to best buy to active your phone, it's the 100% as getting it at Verizon, and sometimes bestbuy can maybe give you "last years" model for even free! (Plus around $50 of tax) as of right now, the iPhone 5s 16gb with contract is $200, when a new iphone comes out, it will be $100 (or even less at places like bestbuy) if you plan on NOT going on a contract, a new iphone is usually $600-$650 depending on the carrier, but if you get one from the previous year, it could go down anywhere from $500-$550.