Rust discount question

http://www.enhancedsteam.com/

Get that extension for your browser (works best on Chrome).
It'll give you pricing for every region, lowest current price anywhere, and also a link to ITAD so you can check pricing history.

Well the largest discount its ever had was 50% off on steam, https://steamdb.info/app/252490/

It was in the humble monthly a few months back, but I have no idea if that'll effect the discount, so I'd expect at least 50%.