cheapest way to winterize sprinkler system?

If you have a system with manual drain valves, shut the water off at the main valve, open the drains and let it empty. That's free. Blowing it out with compressed air is what most people do, but you need a big enough compressor - a small pancake one can't push enough air. And wear eye protection. Hiring someone for a blowout isn't too expensive and you only do it once a year, so for most people that's the cheapest option that doesn't risk a split pipe.