How much my fortnite account is worth?

I have black knight all battlepass skins from season 2 to 8 and 777 wins 3er Kd.

how much do you think i could get?

Probably between 50-300 euro, you have to compare it on eBay or similar sites where accounts are offered. Depending on that, there were very popular skins and if you have them, you get more directly.

Defensively, a few hundred euro if you look up the black knight skin alone is worth 50-200 euro. So you could get at least (if you can find a buyer at all) 500+ euro.