Hmmm. Not terribly likely. Not because of the design obstacle, although that is certainly holding everyone back, but the economics of such a project make the under $100 part fairly unlikely.
Let's assume a run of 200 units, which is enough to get the low-hanging economies of scale and still be within the realm of possible sales volume. Also, this card will do an external color monitor as well as an internal gray scale modification, because, really, it adds next to nothing to the cost and complexity.
If it can be done in a 4-layer circuit board, then the PC boards will cost about $20 each. The FPGA will be $10 - $20 each. The 120 pin Euro-DIN connector will be $4 each. Some kind of Flash on board (for the firmware) is $1. The memory will be...really hard to predict. There are a lot of choices and the market keeps changing. Let's call it $5. The D to A converters will add another $6 - $12 and then the handful of other components adds another $10. That's (taking midpoint costs) $63 just in materials and may be a low estimate (or high if the builder lucks into cheap lots on Ebay). So up front, the maker would be investing $12,600, which he or she will want back.
Oh, but that's just for the frame buffer video card. You still need the gray-scale adapter yoke board. I have no idea what those cost to produce because I do not know the cost of that connector for the rear of the CRT. I would guess in the neighborhood of $20 in materials. And we haven't allocated anything for wiring harnesses which could add another $10 easy but possibly $20. Those molex connectors really add up. And have you ever priced wire?!? It's shockingly expensive.
So, you're looking at $18,600 up front with a material cost of $93 per setup.
Now you could cut that initial investment by paying a lot more for each setup. The circuit boards can be had for $55 each in small quantites (like 4 at a time). So now the total material costs might be something more like $150 per board, but the initial investment could be closer to $500 - $1000 for materials, prototypes, etc.
Now, if you have $18,600 sunk into 200 of something up front, which you must sell for at least $100 each in order to cover your costs, but you're not certain that there are even 100 people who will want one, how do you price it?
Or do you make the $150 version a few at a time? If so, how do you price them?
This example pretty well illustrates the frustrating economics of all these projects.
Even my IIfx 16MB SIMM project would have been uneconomical if I had not found incredibly cheap 72 pin SIMMs from which I could steal the memory chips and had a $500 off coupon at 4pcb.com. If I had had to buy the memory chips loose I could not have afforded to make them and noone would have been able to afford to buy them.
And even then, I had to design a special board which could run off of 16M X 4 chips because SIMMs with 16M X 1 chips are relatively rare and expensive.
And that solution meant hours of extra effort recovering and cleaning the memory chips. That was okay while I was unemployed, because I had the time. Now that I'm happily slinging electrons again, it wouldn't be practicle to make that time for money trade.