I've wishlisted "order of battle" for a little while now and only the DLCs have ever been on sale. Does the main game (with pacific) ever get a discount, or only the DLCs?

I don't know about about the specific game you are asking about. But I do know there are many games on GOG which rarely to never go on sale.

IsThereAnyDeal to the rescue!

According to this esteemed price history site, it was last on sale in June.

Before that it was on sale for 1 day in March, 3 days in February, 7 days in November 2021 and some more frequent intervals before that.

https://www.gogdb.org/product/1744082009
Scroll down to the bottom of the page for the latest sale.

The aforementioned Is There Any Deal is also a fine choice, especially if you're interested in price comparison.

very helpfull, didn't know such a "sales record" site existed, many thanks

The game is currently, for a minor sale, available on their producers webpage.

https://www.slitherine.com/bundle/order-of-battle-world-war-ii-complete

84 euro's and 25 cents for the complete package going for 25% discount.
I believe there are multiple instances of sales going on at the slitherine page. Personally i only bought the German affairs package detailing that front, though i might get into the allies too later on

Other places

Fantical, Morning sun dlc 5,99
https://www.fanatical.com/nl/search?search=order%20of%20battle

HRK , Morning sun 6,01 and U.S. marines 3,79 (though during checkout local taxes might increase the price as does method of check-out)
https://www.hrkgame.com/nl/games/products/?page=1&search=order%20of%20battle