The are about 300 professional bowlers in the USA registered with the Professional Bowling association, that has about 4300 members world wide. Bowling has greatly declined since its heyday in the 1970's when professional bowlers were akin to rock-stars today.
http://www.bowl.com/downloads/pdf/leagueSchedule_12Team.pdf
It depends on what kind of bowling you are speaking of.
However, speaking of the game that 99% of us know (10 Pin Bowling) the pins weigh between 3 lb 6 oz and 3 lb 10 oz and the variance of all 10 pins must not vary by more than 2 ounces for synthetic pins or 4 ounces for wood pins.
Just as a sidenote -- the pin must be 15" tall and 4.7 inches wide at the belly of the pin.