Good high quality paint is economical. It might seen like a lot of money to buy a good low or no VOC paint, but using a paint of lesser value could cost more. Good paint covers and is easier to apply. Reducing the amount of paint needed and lowering the amount of labor needed.
Benjamin Moore make some very good paints and are worth using. All of the good paint manufactures have low VOC paints and many also have true no VOC products. Other brands that I like are Devoe, Porter, Kelly Moore and Pittsburgh.
If you are looking at 20-30 gallons of paint, consider talking to the sales rep or owner of the store. They might be willing to reduce the price a little for the sale.