How can I find the fair price for a game not present on Steam anymore?

check resellers: fanatical, humble bundle
or grey market sites: g2a, kinguin, gamivo

also, steam gifts are more expensive than keys.

To get a rough idea of a games value, look at what it sells for on the grey market. Sometimes prices for removed games might get inflated there, as the sellers hope that someone will, sooner or later, buy the game even if it's too expensive, but it might give you a rough estimate of its value, particularly if the value is low.

It seems that the average price was $ 20 and the lowest was $ 4 (on sale I suppose):
https://isthereanydeal.com/game/lordofringswarinnorth/info/

You should add https://isthereanydeal.com to your bookmarks, it's a very useful site to find a good deal or just to know the average price of a game.

Also, see what other users on Steam Trades are asking for it (but as you experienced first hand, check the last updated date for those posts, too).

Is a very rare game and like melliuc i found on a reseller website in a price around 20 dollars, but if you really loves lords of the rings series you can purchase another delisted cheap game to increase your collection like: Guardians of middle Earth, LEGO Lord of the rings or LEGO the hobbit, they coast less or more than a dollar.

gg.deals