Assuming it gets a case redesign with Thunderbolt 3/USB-C ports and DDR4 RAM, I'd say at least $300. There will be a remaining late 2015 model at the same price it's at now, and it will still be crippled by a 5400 RPM HDD.
They will raise the price significantly, only offer a max of 16 GB of RAM with no way to ever upgrade it yourself, and take away all of your ports, and shorten your power cord by half. Sounds ludicrous, but they just did it today to MBP users...