how do I calculate the transfer price here? what's the correct answer?

how do I calculate the transfer price here? what's the correct answer?

I thought it was $8.14, because:

selling 30000 units internally = 5000 units of sales loss * 1.5 (contribution margin) = $75000 loss.

This amount should be spread thru 35000 units sold internally > 75000/35000 = $2.14 opportunity cost charged per unit

Therefore 6 + 2.14 = $8.14

Am I correct?