Expensive/cheap doesn't really matter, as long as you enjoy the champion. I wouldn't recommend buying expensive champions blindly until you have a few that you can play while you farm more IP (whether they're expensive or cheap doesn't really matter), but it depends on the player at hand. The big thing is to know what you're getting yourself into. If you're okay playing free champions if you don't enjoy your first purchase, then by all means pick someone up. The problem is when players blindly buy someone hoping to like them, find out they don't like them, and then get frustrated when they don't have anyone to play.