Diggs was a trade not FA, but I generally agree with your sentiment, especially this year when the cap will not rise. For WRs the league has shown in recent history that you can get contributors immediately in the mid rounds of the draft. There is little point in my mind giving any one of these FA WRs a Mike Wallace kind of deal when you can pick up someone in the those mid rounds who will play on a rookie deal. I don't see a world beater in that group, certainly no one I would pay what they are asking for.
I would personally focus FA money on resigning our own players, such as extending Ogbah, and if there is money left over filling any holes we have on the D, so we can focus O in the draft.