Graduate programs in medicine are locating in hospitals and receive federal funds (GME) that pay for residents. Dental programs in hospitals (GPRs, OMS, peds) receive the same funding) so they have money to pay residents. The funds also give some extra to pay faculty.
Dental schools do not receive these funds. (some did but lost it a year or so ago) therefor no federal money for residents or faculty. To run the programs, which still lose money they charge tuition. To be competative some try to give a stipend.
It a long story why the fundingh is given and goes back to when dentistry wanted to stay away from insurance (medicare,etc) and now all insurances are trying to cut back.
The big relation to pay is where the program is, what funding they have, and not necessarily the quality.