Why are you attempting a question for which you do not have an answer? You should be using a Revision Kit from one of the ACCA Approved Publishers – they have answers and explanations!!
You answer is not correct. The expected output is 1,000 – (10% x 1,000) = 900 units.
Therefore the cost per unit is $2,700 / 900 = $3.
This is all explained in detail in my free lectures on process costing. The lectures are a complete free course for Paper MA and cover everything needed to be able to pass the exam well.