in what extent do I buy cards? how much is the payback? and what are the pitfalls?
well, currently, if you have AMD cards it is better to mine xmr or aeon - you'll get more profits from there
if you have nvidia - there is a bunch of alts to choose for mining
but there is another option, just but btc and watch how it rising to the moon..
In cost wise AMD Radeon cards costlier than other cards but you can still mine with the Nividia GTX 1080ti to mine any altcoins. I have a got the quote for the one rig cheaper than AMD cards contained one.
If you have to go with Nvidia you can able to mine Monero, Zcash and Ethereum coins with the better to choose if you have a standard AMD 470 cards.