Id love to do this kinda thing for a living,
Reminds me of my 2600 clone experiment, with a 6502, tia, 6532, ay-3-8910, atmega8515, and 64k of ram, 32k +128bytes mapped the cpu, other 32k ported via the 8910, use the avr as a coprocessor, if utilidzed, it will handle the tia, the avr would run at 21.47727 mhz, 6x the tia clockspeed (for me tis 3.579545mhz), and 18x faster than the cpu, which runs 1/3 of the tia speed so everything still can easily be synchronized, its a very experimental computer, its at a point now were its better to ditch native 2600 compatibility, but one would still easily be able to port the existing 2600 codebase......
All the parts execpt the tia are still made, if i were to actually sell this (working), depends how cheap i can get 2 layer pcbs for, i aint etching and drilling all that, and spare tia chips,
I could possibly get assembled units with a bom of $50-$80usd per unit, just for the board, you may or may not get a case tho, too bad!, You at least would have svideo and composite built in tho..