Well, according to ol' Wikipedia, in 2009 the Cell was at 45nm and the RSX was at 65nm. In 2010 the RSX chip was shrunk to 40nm. Since then neither chip has received a die shrink.
Having said that, as lithography processes mature and are replaced by smaller geometries the price of the previous sizes tends to go down, so its not inconceivable that the Cell and RSX chips have decreased in price from 2010, though it would be a minor reduction. Conversely, each new lithography process is more expensive than the one before (per wafer) due to escalating R&D costs. The effect of this is that eventually at small enough die sizes the R&D and manufacturing cost at smaller processes outweighs the cost saving of the smaller die. Unless you are selling gigantic volume of processors (think Intel or the various ARM SoC designers), its not cost effective to keep shrinking the process of the same design, which is why plenty of semiconductors are still made today at >180nm sizes.
Regarding combining the Cell and RSX chip into a single die, that will be expensive due to the Cell chip being on SOI (silicon on insulator) and the RSX chip being on bulk silicon. To combine them would require the Cell processor be completely redesigned to be fabricated on bulk silicon, and at this late stage in the PS3's life I don't believe that they will sell enough die's to justify the cost. MS and IBM did it for the Xenon processor in 2010, but back then with another 4-5 years of 360's life to go it would have made financial sense to. I think the ship has sailed on that one for Sony.
