A few years back I had taken several rolls of 135-36 color film to the convenient pro lab that did quality work. They charged $3.50 or $4.00 a roll for developing and did quality work. Then I took 8 or 10 sheets of 4x5 color sheet film in for developing and they charged me the $3.50-$4.00 per sheet to develop them. I wasn't paying for the film surface/chemical requirements but per load into the processing machine. I did not like the clip marks that ranged from the rebate edge of the film to 1/4 inch into the frame from the dip and dunk tank clips so I resorted to processing my own film.
Also its nice to be able to select the format you want for the subject at hand and still use just one camera.