In simple: That much you can afford to lose.
Imagine worst possible scenario could happen in future: A total collapse.

Say a major flaw is found in BTC (ETH) protocol. Exploited so much eventually making coins worthless (nobody is buying anymore and you are unable to cashout into fiat).
aka
your crypto holdings value turns into $0.

Put in only that much so you can say something like: "Hmm, that was an interesting experiment. Shame it failed but I can move on." should the above happen.