Justin Bieber was caught speeding in the valley and it's cost him thousands ... which is just how he planned it.
Biebs was down for a little late night go-kart action Wednesday night for a bud's b-day ... and dropped a few grand for an after-hours, all-access adrenaline session at MB2 Raceway.
We're told ... JB arrived with a dozen friends -- including Hailey Baldwin. We're told he dropped $2,700 ... the cost of burning rubber for 2 hours.
The staff gladly stuck around ... Justin threw them an extra $500.
Go Biebs Racer ... Go!