I don't exactly see the problem with her cost. Most of the time you will deploy a soldier and anticipate more heavy forces down the line when you have set up a decent team. If you need to quickly deal with enemies you'd deploy archers regardless. Heavy Armours cost a decent amount overall too so waiting some extra time to deploy Nadia instead is fully plausible.

And as others have stated in many cases she won't need a blocker since she will outright murder anything that gets into her range. To give you a similar example - would you be deploying gellius because you are worried Spica will let those purple goblins that she 1-2 shots through? You'd most likely just place her and giggle at the goblins getting slaughtered. Nadia works the same way but for tougher enemies.