I never heard of any BTC mixer with monkey on their domain name I tried to search but I couldn't find any.
You might be talking about Banano mixer? Someone mentioned it here on the forum but it's not for BTC it's for Banano.
Source here
https://bitcointalk.org/index.php?topic=5165788.msg51947827#msg51947827Question to you... Let's assume that is true and that it was indeed in the FAQ. My question, to you, is... such vital piece of info, why put it there and not clearly on the page of purchase?
Most of mixer tells you first on the home page how much the required amount before you deposit so if the mixer you use never tells you how much even on their faqs it's their fault and they should send your BTC back to your wallet minus the transaction fees.
I suggest check your browser history and maybe the mixer you talking about is still there and post the exact domain name here so that we can check if they never told you about the required amount.