How Much should I charge for XXXX
There are many different ways you can charge for irrigation, the two most common that I hear are by the head, and by the zone.
If you charge by the head, factor in the cost of parts and labor for each head (Head, Swing Joint, T, 20 ft of pipe + Labor). Usually people add a flat rate charge for the back flow, timer, and mainline/wire installation.
Other people charge a flat rate per zone. This would be done by averaging the size of your zone, and calculating its cost.
Personally, I find that per zone charging is unpredictable, especially if you end up with a zone that has 25 spray heads....That is going to take a lot longer than a zone that has 5 rotors.
Finally, know your market. Just because Contractor A can sell his system for $8500 in one state, doesn't mean that is the going rate in your area.
The best advice I can give is measure your job before your bid, check soil conditions, look for obstacles that could be headaches (tree roots, sidewalks, utilities, poor machine access) and try to factor in the additional labor they will cause into your bid.