Skylake is showing in some tests about a 5% gain over the devils canyon refresh line up, so there is some gain but not a hell of a lot. The idea that DDR4 is horribly expensive is pretty much false as I said earlier. If you look at the high speed DDR3 vs high speed DDR4 (using G.skill as an example) you'll find the DDR4 stuff being cheaper.
If you only have to upgrade your CPU, then no the cost difference isnt really worth it. But if you have to buy a new CPU, and motherboard anyway, I'd look in to what the actual price of the memory you would want is rather than go on false assumptions of cost. And with the Black Friday sales coming up, you could very easily get in to either system for much cheaper. Either way, a Haswell i7 or an Skylake i7 will do what you want it to do with flying colors.
If you wanted to get in to a workstation class board (not X99 but something like a Z97-WS) you could use an i7 and then Quad SLI the hell out of it but if you have that kind of money to put in to a rig, the cost worries of DDR 4 are nothing.
As for your SSD/HDD and videocard, they dont care if you go Haswell or Skylake, they'll work with either.