Don't know about it being a licence to print money. At best a DPE can do one checkride per day. In practice there just aren't that many to be done. I would not want to try to make a living as a DPE (though people do, along with some instruction).
It's just like flight instructing. Round here most instructors charge $60-80/hr. Not bad - IF you can get 40 hrs/week of instruction. But you can't. I don't know how many billable hours per week a full-time instructor gets, but I'd imagine that even 20 would be unusual.