Why? Each decent championship team usually has at least two or three premier league quality players. What's wrong with taking them? Same with relegated teams. If the boot was on the other foot and we were relegated, would you have judged West Ham for signing Mings, McGinn or Grealish?Other teams will up the prices, as they did last summer. I don't think we'll pay over the odds, or only in exceptional cases. £20m for Championship players? Nope. In general, buying Championship players is even worse than buying relegated PL players.
I'd look for players from the Spanish and French, and maybe even Italian, leagues, where clubs have a lot of financial troubles.
Taking the best of the championship players and relegated premier league players has many off field advantages. Generally easier to settle in, no language barriers etc. We took Luke Young, Milner, Ashley Young, Darren Bent, Fabian Delph etc from lesser sides and they all went on to do very well for us.