Population is important. Whether you think it's "worth it" or not depends on several factors: your treasury and per-turn income; your population and pop growth; your typical army composition; etc. Yes, the Dorwinion Bowmen cost double the other archers. I'd be asking for a raise if my boss wanted me to fight in a melee too.
We're clearly not above tweaking units here, so if you have suggestions, let's hear 'em. But keep in mind that we're trying to hit a few notes with the unit in question:
-its (pseudo)Dunedanic heritage means it will have a smaller unit size, but
-it will have better armor/higher defense, and thus
-it will have a higher cost
As for the cavalry, IMO Dorwinion actually has some of the more useful cav in the mod. The Rhovanion Scouts are great for the cost for all the reasons athanaric listed above. The bodyguard cav fills an interesting role in Dorwinion's roster: think of it more as "mounted infantry" than true cavalry. That is, it enjoys the mobility of a cav unit and the greater staying power of infantry due to its armor, sword and shield - but it doesn't have a great charge and the small unit size can be a liability.