It's a pretty prescision little device. The tricky part would be drilling the holes in the sprinkler arm. They are angled and that's what causes it to spin. It's a very light, balanced arm.
If you go check one out at the HBS, you'll have a feeling for what's involved. I can't really imagine you could get the parts and build one for less than $15 (maybe they're cheaper than this), but who knows. You'd need some really delicate drill press or something to drill those holes in a tiny little pipe.
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit