The prices quoted in the first link are about right for a respectable dropzone. I would be careful with anyone going much lower than that--not necessarily unsafe, but likely to be not much of an enjoyable experience. Each dropzone has its own character, finding a fun and friendly place (and safe, but most of them are) should be a much more important consideration than price in your search--seeing as it's a gift for your son, it would be a different matter if it was say your mother-in-law we were talking about.
