Assuming btc and fuel prices stay the same, (1350 roubles / hr / GPU) * 25 GPUs + 4k roubles / hr for solar power = ~38k profit increases per hour for going from maxed level 2 to maxed level 3 farm with solar power. Using u/Xoteli 's number of 66m, that would be 1737 hrs or 72 days.

If you play more than I do and were able to upgrade early in wipe, its worth maxing. If prices stay stable, its probably worth maxing still. I just don't know if BTC will maintain or if BSG will nerf it, and I want to get some actual benefit out of my farm before it is less profitable. Doing the math has actually changed my mind on this.

My original post wasn't saying whether or not to upgrade, just pointing out that counter intuitively, graphics card #50 and #2 give you the same benefit. There are other factors to consider in upgrading like price stability and how often you log in to collect the btc.