Sony spent a lot on developing Cell, and especially on developing Blu-Ray. They can use both of those technologies in a new console without suffering anything like PS3 development costs.
Sony has been paying for die shrinks of Cell and RSX this entire generation, as they've been lowering the cost of the PS3s they are selling. Cell itself was designed to be a scalable architecture, with support for multiprocessing (i.e., multiple Cells) from the beginning. They could put a 28nm next gen Cell chip with 2 PPEs and 16 SPEs and have something decent, or they could do 4 PPEs and 8 SPEs (for backwards compatibility), or perhaps they could even take handful of Power 7 cores along with the 8 SPEs to get good branchy behavior along with the vector processing of the SPEs for backwards compatibility.
None of that should cost anything like what it cost to develop the first Cell chip.
As far as Blu-Ray, that was *expensive* when the PS3 launched. Those 405nm laser diodes were hard to come by, with really poor yield, and they cost a *lot*. Nowadays, they could put in an 8x BD drive "off the shelf", and get better performance with far, far lower costs than they had to come up with Blu-Ray in the first place.
With such a system, they could bring forward all of the software they developed in coming up with the PS3, as well. PS2 had basically no operating system, so Sony had a lot of software development costs to bring PS3 out. All that is paid for and ready to go if they wanted to go with a next gen Cell and Blu-Ray.
The GPU is going to be something modern and fast from AMD or Nvidia, so that's not development costs they'll have to incur, either. And remember that consoles don't have to worry about driving 4k monitors at higher than 60hz or anything crazy like that. Just a nice, simple 1920x1080 x 60 frames. That's easy for modern GPUs.
All in all, Sony should be able to spend far less on development costs while still fielding a very powerful next gen system. They just have to take advantage of the very significant investments they have already sunk developing PS3.