It’s one thing to get a Rod Stewart impersonator, or a Bon Jovi cover band, to play at your daughter’s sweet 16 party, but what’s the real thing cost? We ask because Patriots owner Robert Kraft is throwing an invite-only bash at Gillette Stadium Wednesday, and he’s booked Sir Elton John to entertain his guests. Kraft and the Rocket Man are longtime friends, so it’s unlikely the Pats poobah is paying John’s usual performance fee, which is believed to be north of $3 million. But you don’t have to be a billionaire to get a big-name act to play at your kid’s bar mitzvah. We spoke to a national music broker, who gave us a breakdown of the tiers of talent and what they charge for a hits-heavy set lasting between 30 and 90 minutes. The numbers below are performance fees only and do not include additional expenses such as airfare, hotels, ground transportation, and security, which can add significantly to the bottom line.