From a cost perspective, you want to use 1" pipe... if you can.
1/2", 3/4" and 1" PVC are the most common sizes used/sold (at the big box retailer). Since these are the highest volume (in sales), they have the best prices. Therefore, you are getting the most bang for the buck with 1" PVC. Once you go above 1", not as much is sold. As a result, the 'discount' price you get at the big box retailers isn't as good for this lower volume item.
But as Boots points out, you've got to do the math. If 1" PVC pipe at your designed flow rates creates too much of a pressure loss, your irrigation system won't work correctly and rather than saving money using 1" PVC you will loose money because it will all be a waste.
If you need a table, you can find one
here on page 4 for Sch 40 PVC pipe.
If you use 1" pipe, and your system is designed to run at 12 gallons per minute (assuming your system can supply that much), then you are going to loose about 7psi on a 200' run (3.36 *2). Increase pipe size to 1-1/4 and pressure losses are <2psi. But if you design for 9gpm, then your 1" pipe will only loose 4psi on a 200' run.