I don't think you can really try to point cost a unit by saying if its bigger it cant get as many attacks on a smaller unit so should be cheaper. Because thats only valid against a smaller unit. Against a not-small unit, they'd be getting all of their attacks.
Also with something like say skeleton spears which have a 2" reach I can get 25 of them to attack against 10 models and depending on setup even more.
EDIT: also even if a small unit of 10 models keyholes its way in so only 10 skeletons can attack, those 10 skeletons would still have 3 attacks a piece if hte unit was 30 or more, so they are getting 30 attacks to the unit they are fightings base attack value (whatever that is).