You'll catch the first two games on sale on Steam typically for 50% off. I think they skipped the latest sale because a) they knew people would be hyped for the new game and b) they actually did a Koei weekend sale last month where the first two games in the series were $29.99 each. If you've waited this long to play these, I'd say you can hold out a bit more because there were a total of 6 times in the past year that the games were on sale (for a limited time).
isthereanydeal.com
50% is about the best you could hope for though, they're unlikely to sell it below that. The newer games are actually being sold and discounted on third party sites now at least, so you might have some luck there. For example, this new game is on Green Man Gaming for about $10 off:
Title No Longer Available at Green Man Gaming
www.greenmangaming.com
It's worse for physical copies though, especially on the Switch. It's like they only make 1,000 of them or something. They usually sell out everywhere within a few weeks, and never get reprinted. Ebay prices for used copies usually go for $80 and up. Sealed copies can go for a lot more. That's not normal for a game series that's still releasing new games.