When you look at the cost of Cell you need to look at the development cost in addition to the manufacturing cost. I don't know the numbers, but Sony spent far more developing Cell than they would have with a more off the shelf part.
From memory, the three companies were willing to spent $400 million developing Cell. Before STI sat on the table, IBM already has some form of the PPU and Toshiba already have the early SPU. I guess Sony brought in the vision of putting PPUs and SPUs on a single chip with ringbus and DMA engine as well as north and south bridge. I think Sony only went to IBM for the manufacturing process. If not it would just be Sony and Toshiba like the PS2 Emotion Engine. Maybe with ARM or MIPS for the PPU.
So I don't think it was that expensive relatively in developing Cell. NV spent far more money in developing the architecture that becomes RSX and their 6000 and 7000 series line up.
In a way Cell is using off the shelf components unlike GPUs from NV or ATI.
I think more interesting question is why Sony went with NV RSX ? That must be desperation.
My theory went like this, Sony followed the same development timeline for PS3 as they did with PS1 and PS2. That is they had developed a really powerful rasteriser with no processing units that can make it into a functioning GPU. So like PS2 they need a CPU that can provide the FLOPS. So again like Emotion Engine they went to Toshiba for some DSPs and later to IBM. At this point Sony is investing in Cell development and the manufacturing of their GS successor.
At some point Xbox 360 announced and launched with 512 MB of memory, first signed of trouble, Sony only intended PS3 with 256 MB, since that's what they had ordered. Later IBM only managed to put in a single PPU and 8 SPUs into Broadband Engine, far less than what Sony needed to power their beastly rasteriser. I think at that point they had Toshiba GPU and NV GPU for consideration. They picked NV to meet their FLOPS requirement for PS3 and they can attach another 256 MB to match Xbox 360, problem solved. Only later, they discovered RSX can't emulate PS2 and it went downhill from there.
The early 360 ports look bad on PS3, it took years before it's near parity. I think if Sony had dropped Cell as well at that point and went with some other CPU, PS3 would never catch up to 360.
Not too sure if PS4 will be PS3 done right, ie one 2 billion transistors chip dedicated mostly to SPUs for FLOPS and another 2 billion transistor processor dedicated to raster. Looking at Vita, it seems the current Sony is more concerned with ease of development than crazy architecture.