I bought 3,000 lady buys for I think $11 off amazon. Shipping was the most expensive part; I think it was $15 for shipping regardless of if you purchased 500 or 3,000. I spray weakly and I didn't have a pest problem that I know of, but I figured for $25 it would be a cool experience and have them look the plants over for me before the stretch.
I sprayed the plants with water and then released in the evening. Was really cool, I let them go at the base of the plants and they would just walk up the stalks and get to work lol. 24 hours later probably 90% of the ladybugs were gone, after 3-4 days they were all gone.
I enjoyed the experience but like pahpah said its probably best to get a spray.