You can't just compare skin RP prices and champion RP prices,like that.
If that was the case,then every 1350 skin would require more work than a full champion,which obviously isn't the case,as the 1350-skin doesn't even come with a completely new VO or animations.
The legendary and ultimate skins (the one for ~1800 and ~3200) are the only ones who are comparable to new champions in terms of quality(though the older ones are obviously outdated by now),however I wouldn't say they could have just made new champs instead because not only is the skin team a different one than the champion team(so i doubt they would be that good at making champs instead) and skins don't face certain problems that new champs do face(they don't have to worry about the kit that much and the character itself).
Also by your logic they could simply stop the production of skins entirely to make more champs.