anyone knows if it’s cheaper to get random domain, just for personal stuff, this way

I think it’s usually more expensive that way. What I’ve seen is that these are usually domains that might be of special interest. This is also the case in the one that I have seen. (In contrast to auctions of physical things like on ebay, that might have aged already and thus are cheaper.)

how are the renewal fees set, are they related bid made

That’s what I would like to know too. Either that, or the prices are the normal fees of the chosen registrar. I don’t know which.

The problem with that is, that not all instances use “Lemmy” or even “feddit” in the URL.