TLDR: 1 gfx = good! more = not as good as you think.
Bitcoin farms are all the rage, getting 0.5 M per day is really nice. However ignoring the 0.2M / day fuel cost and the build cost I think the value of GFX is overvalued. It constantly hovers around 2x 0.2BTC price which makes calculations rather simple and now its ~1M i did some back of the envelope number crunching.
The wiki states that payback time of 1 card is ~ 1.7 days and 2 cards is ~3.25 days. While this is true if all your BTC money is spent on GFX really what you want to know is how long the incremental increase in BTC production pays off the additional GFX. This is not as simple as 3.25- 1.7 .
Instead, taking the BTC value formula from the wiki and working with roubles/hour you can see that the first GFX is 1.7 days and the second is 37 days. The 3rd card is 37 days and every card thereafter pays for itself in 37 days. Note: that if you chuck 5 cards in it isn't (37×5 =) 185 days but 37 days as they are incrementally paying for themselves not cumulatively.
Since gfx is currently ~ 2x btc I think this is a better approximation for payback time. Knowing this you can do a couple of other calculations.
If you had 50 million and dropped it all on the cards at once (ignoring hideout upgrade costs etc) we know we get 27 days of 0 profit to pay it back before getting 1.6 million / day . In that same time 1 card would have made ~ 14 million. That's a month of that upgrade doing nothing useful.
Now weighing up how long the wipe will last ( 5 more month? 10?) whether you want that investment or not is up to you. Do you want the money now for raiding or are you happy doing money runs now to raid late game in the future?
I've dumped my working here ( sorry for spelling)
obviously if you die with a GFX in your container pop it in there as it better than vendoring it, but I don't think gfx = 2x BTC is a realistic price for them. If I had to guess I would say ~10 day payback would be more likely so gfx = 0.5 btc would be a more stable price to expect.
Glad to hear your thoughts!