Things to consider:
- How hard/soft your water is. Softer water = less detergent required, hard water = more detergent, possible white film on dishes. You'll have to experiment.
- How much pre cleaning you do. DON'T DO IT. Any new machine should be able to handle cleaning dishes, and some use sensors to determine how vigorously to wash dishes, and if there is no dirt to sense because you precleaned, it will do a crappy job of washing.
- How much you care. I am a terrible lazy asshole, I don't give a f* if I have calcium deposits on my glasses, or if a dish occasionally has a piece of cheese that didn't get washed off.
- How full it is. Don't run the dishwasher unless it is full or starting to smell. The manual will have recommendations for loading it. Experiment with maximizing loads!
When left to my own devices, I buy the cheapest powder detergent from the grocery store (my mom sometimes gets me fancy name brand stuff with a weird points thing from work) and it seems to get things clean, albeit with a white film from the hard water (I just do a vinegar rinse every once in awhile if I'm having guests or I get an attack of caring). I never put any soap in the prewash thingie and I don't fill the main soap compartment unless I'm washing lots of really greasy stuff. That said, my water is pretty hard, so I don't skimp either. For me and my roommate, with a low to medium cooking load, a single large box of cheap store brand powder lasted about a year. I run the dishwasher 1-2x a week.