The best answer to this question is that it depends on where you live, what type of equipment you plan to install, and whether you will do some of the work yourself. But the following will help you at least ballpark what you ought to be paying for an automatic sprinkler system.
Do you plan on having a professional contractor install part or all of the system?
A basic rule of thumb is that a sprinkler contractor will price things out so that he has 1/3 of the total bid price in materials, 1/3 in labor and overhead, and 1/3 in profit. That leaves enough room so that he can still make money on the job even if some of the site-specific variables like a hard to find mainline, lower-than-expected pressure, or hardpan soil throw him some surprises. You should be able to buy equivalent materials at a home improvement store for about 15-20% more than the contractor will pay at a contractor supply store, but you will need to factor in rental of trenchers and whether you can install the backflow prevention yourself, so you should figure your total cost of installing the system yourself will be around half the average of three competitive bids. A really simple way to estimate the cost of installing the system yourself is to use the Orbit Sprinkler System Designer.
If you need a quick and very rough range of where you might expect bids from professional sprinkler contractors to come in, you could use a range of $0.50 to $1.25 per square foot of area to be watered and probably not be too surprised. Just keep in mind that small properties will tend to be at the higher end of the range.
This is the first of a four part series that will ask the questions to help give you a good idea of things that will drive the cost of an automatic sprinkler system up or down. Some are things that you can choose and some are things that your specific region, climate or building codes will drive.