It's okay to be profit-driven. I don't see why this is a problem. The financial stability of a field is important because it takes money to maintain them and no one wants to do it for free. I don't see making a few bucks off of it as a crime.
Field BBs CAN be okay if the field listens intently to the players and brings in a solid brand or two with lots of choice... And you'd have to be willing to adapt to meet the needs of your players... This is also something you can't gouge on. Tack maybe an extra $1 to $2 per 2000 and you should be alright.
It's important that a large portion of the profits goes back into the field. I can't tell you how painful it is to play loyally at a field for years and watch exactly 0% of the cash given go nowhere into maintenance and improvements.
If you're going to build, contact the established community and get field-specific feedback for design. Those guys will keep coming back if they're feeling like they are part of the place.
That's what I got.
Age Verifier - Lower Vancouver Island
Brotherhood of Nod - Nod Prime || Vancouver Island Airsoft League - President