Bié ràng tā qiāo nǐ de zhúgàng, shìchǎngjià wǒ qīngchu.
Don't let him gouge you - I know the going rate.
Tips
history
A Qing-dynasty origin story: opium smugglers hid their goods in hollow bamboo poles on boats, and river officials would 'tap the bamboo' to demand a bribe - otherwise they'd 'inspect' and uncover the contraband. Tapping became slang for extorting under a pretext. 《官场现形记》 chapter 17 already uses it casually.
usage
The object goes between 敲 and 竹杠: 敲他的竹杠 ('to rip him off'). Very colloquial.
