water amounts for use in a greenhouse

i have not farmed in a greenhouse much before. i have an opportunity to rent 2 30' x 70' greenhouses that belong to a non profit that helps farm startups (and generally promotes local ag).....
only issue is the water district they are connected to has very expensive rates.
40$ per month for service hookup &
.011c per gallon.
im wondering of anyone has a good idea of how much water might be needed per square foot...
im planning on growing lettuce heads in the spring fall and winter as well as tomatoes interplanted with basil in the summer.
i will be using drip and organic (not plastic) mulch to save water.
any thoughts or suggestions???

My irrigation system is designed to deliver one inch per week, which would be .6 gallons per square foot per week. That's for my open fields in the desert.  

My experience in an unheated solar greenhouse is that in the winter very little water is actually needed. Most of the moisture that is transpired by the plants or evaporated from the soil stays in the air, or condenses on the glazing at night and falls back down. And it's cool, and there's no wind. As mentioned by Eliot Coleman in his excellent books about growing vegetables in unheated greenhouses in a cold winter climate, it becomes possible to water almost not at all, but then if you soil is rich, the plants may find t too rich and get aphids, so it's good to water anyway once in a while. Once every week or ten days is probably what I do. I mulch, as it's a home garden and not too huge.

For summer I remove the glazing entirely and my outdoor summer is like Joseph's high desert, and my water needs are probably similar to his in summer.