Nav coin recently released an updated roadmap for this years development and soon they will be introducing valenodes which are masternodes running on their new Valence platform: https://navcoin.org/project-roadmap/

However, it does not state exactly when to expect those to be ready and it also does not specify how many coins you will need to create a valenode. My best guess is that it will be 10,000 NAV per valenode which even at the current price will be a lot, but I feel that Nav is a sleeping giant already and these guys will most likely be around for the long term.