It does make sense that preschool is going to cost less than daycare. Preschools will have the children on the premises for a much shorter amount of time. However, the price won't be proportional to the time; chances are the preschool will offer a much larger amount of field trips, lessons, materials.
I have spoken with preschool owners that run part-time programs that charge about $100 / month for 2 days a week at 3 hours per day. On this same equation, a full time preschool would be about $500 - $600 per month. This is probably a mid range.
There is a lot to take into account when setting a preschools prices: supply costs, teacher salaries, facility rent, class size, activities...... and the list could be much longer.
As long as costs are justified, the children are learning, and the parents are getting a great value from your preschool, there should not be a problem with everyone receiving benefit from the preschool.