They've had sales too (Luiz, likely Diaby, Kellyman, Tim) to help with FFP, and will likely have to make sales next June, but like you say their wage bill must be horrendous, even what they're paying Digne would probably make him our highest earner and he's a backup left back. And big name "free" signings like Kamara or Tielemans will have commanded a huge wage to persuade them.
They've thrown the kitchen sink at European / CL qualification and fair play, it's worked, but if they don't sustain it every year they're storing up the same sort of trouble we've just had. Can't fault the ambition though.