So after doing a little digging and taking a look at the possible earnings for both Frasier and Niles I'm still not sure as to who can actually afford the standard of living shown in the show. Frasier goes a period of unemployment yet there is no mentioning in the show of him having to cut back on his spending until a later season when he is reemployed. As a medical professional with his own radio show it's possible that Frasier would have had to take a pay cut in order to be hired by KACL yet additional money can be made due to sponsor's and syndication however that takes time.
Niles on the other hand seems to have a higher standard of living as evidenced by the look and size of his apartment (He has three floors) and the gag of Martin being lost in his apartment looking for a way to get back to the living room. Frasier even appears shocked at the amount of rent Nile's is paying when he has to move out of The Montana and into the Shangri-La. Both make good money but is it really enough?