No. Nexus are not similar to moDB, because Nexus is part of the modding community and the current owner of Nexus is Dark0ne aka Robin Scott. Furthermore, I doubt that moDB offers premium membership.
Anyone can create an account on Nexus, so membership on Nexus are free. Let me guess, what you're referring to is about the premium membership that Nexus offers and yes it seems exspensive, but consider what you get for that kind of money is actually generous, I mean instant download and no ads whatsoever. Of course if you spent all your time there.
Whether an uploaded mod remains indefinitely on Nexus is up for the author to decide and not the community.
Here is one former member on Nexus who simply told Dark0ne that he didn't want his mods to be available on Nexus and that happen over 4 years ago.
Requested
Granted
Another thing that's definitely different between modDB and Nexus is that Nexus now have automatic virus scanning for all uploaded mods there, but to my knowledge that only applies for new hosted mods while some older mods aren't virus scanned.
Should a mod(s) on Nexus not be virus scanned then any member (guest?) on Nexus will know for sure.