... just buy it... it helps the developers... and its not that expensive for all the game and access to mods you get...
20 bucks is not that expensive for a game. I know on Steam that's like every game but most of those games suck nuts.
Buy it now. The price rises as more stuff is added to the game. It was $15 when I got it three years ago, it may be $60 three years from now. Get in while it's good.
If you buy it through humble you get a steam key (so the end result is the same as buying it through steam) and save a couple bucks. I don't think the game has ever been on sale, but it is a great value and by video game standards it is very cheap, so it really shouldn't matter.
i would HIGHLY recommend getting the humble bundle. Its 19.99 and you still get a steam key. Its cheaper than getting it on steam and you still get a steam key emailed to you. I was lucky enough to buy this game for 15 dollars back in the day though. Bottom of the site (https://www.beamng.com/)
You can have a look at Steamdb.info and look at the price history, and get a rough idea on when the game goes on sale