Prepare for some hazing. Pricing per zone will get you too many ups and downs on profits. You really need to do a time and materials breakdown for each job. I know, many guys bid per zone, and I have done it many moons ago. But I learned from all these old geezers here and have become a better irrigator for it.
I could have a 4 zone system using 300' of pipe, in sand, 3/4" PVB, in and out in 6 man hours. Or I could have a 4 zone system using 900' of pipe, in rock/granite, 1" RP, taking 20 many hours. You do the math........
If it's cookie cutter subdivision homes you MIGHT be able to come up with a simple formula.