15 minutes seems a bit shy of a show for a paying client unless it is just for friends / family. For your first show I would shoot for 25-30 minutes and charge $50.00
After you have about 10-20 paying gigs under your belt, then it would be ok to bump it up to $100-$150 for a 40 minute show.
I currently charge $100 per hour of strolling magic and for a birthday party gig I charge $125.00 for a half hour show followed by some balloon animals. If they want a longer show (45 minutes) and balloons I up it to $150.00
This also depends on the demographics of your community and what others around you charge. You do not want to go around doing the $50 shows forever or you are undercutting everyone around you and you'll make enemies very quickly.