The only consistent way to get playmakers at a reasonable price is to draft them.
The best you get in free agency is overpriced mid-ranged guys unless a malcontent like TO hits the market.
Generally, you can find role players in free agency, but not playmakers and diffference makers. If they are still valuable, they are not on the market.
The Bills need to get a dominant MLB for the defense to operate most efficiently. If they think Willis is the guy and they need to bypass SF to make sure it happens, then they should do it. The loss of a player in the 4th round who is most likely a backup is worth it.