The question isnt how how many youth league bowlers become adult league bowlers. The question is how many youths who dont try junior leagues become adult league bowlers.
The attrition rate is high for most youth activities, but the more kids who are exposed to the organized game, the more who will stick with it, no matter how low the percentage. I dont really think it is about cost. Junior bowling is a pretty low cost alternative to many other youth activities already.