not really, the cost of the gpu is not only the core, but the routing of the bus on the mobo, which is quite expensive, there is a reason that there is only 1 gpu using 192bit and 2 using 256bit, most use 128bit, like the 650m
aside that notebook parts cost more, they are higher binned cores
No, a 192 or 256 bit bus require a larger chip because they need a larger memory controller on the gpu chip (making it more expensive for the gpu chip). The data connection between the gpu and the cpu is PCI Express (bus width does not affect this). The only thing that the bus affects is the data width to the vram chips (which can increase complexity and costs).
The reason few gpu's use wider buses is that mobile gpus are often too weak to require more bandwidth. The 650m does not need anywhere near the 80 GB/s that it is given. Even the 680m with a 256 bit bus only gets about 96 GB/s (and is generally about twice the speed). For comparison the 650 ti (desktop) has a 128 bit bus and gets similar bandwidth to the rmbp 650m but is considerably faster.
The actual notebook chip costs more but the fact that there is no PCB + fans + etc. decreases costs quite a bit. For example EVGA buys the GK107 chip from nvidia for some amount of money then buys vram from somewhere else and a pcb and fan from somewhere else and then manufactures a graphics card that they sell for a profit to newegg which sells for a profit to you for around $100. Cutting out EVGA's and newegg's profits + associated costs (shipping, warehousing, handling) the actual 650m + vram costs much less than $100.