For bitcoin you need special mining gear and setup. Using GFX is not really worth it anymore, i think. But other crypto coins like Etherium are very popular right now. Bitcoin has behaved like a bubble. Those who sold it in time got rich, others are in tears or still rich. And governments try to get a grip on it.
Too bad GFX prizes are suffering from webshop pulling a fast one on it. Nvidia already stepped in and wants them to sell only 2 cards per customer. They even have made special BIOS for crypto miners, so did AMD. It is big business for them, but at the same time they see the gamers market suffer. People who want to upgrade now, are confronted with ridiculous price raises, depending on the type of cards.
In that regard, it is even more lucrative to buy a Titan from Nvidia directly, than a GTX1080ti
A GTX1070 will perform at a similar level as 2x GTX780ti in SLI or 2x GTX970 in SLI.
The GTX1070ti is almost as good as a GTX1080.
Every niche in the market is filled now. And they will not launch new cards as quick as previously.
I might still have a GTX780ti laying around somewhere...