make versus buy

Hello everyone,

Can someone help me to understand the solution to the question?
The question is:

A company plans to build an extension to its factory. The estimated costs of carrying out the work inhouse are as follows:
Materials – 58,500
Labour – 32,800
Additional overheads – 17,200
Allocated fixed overheads – 8,200
Total – 116,700

An outside contractor has bid €110,000 for the job.
What is the net gain or loss if the company does its own construction work?
A €35,200 loss
B €27,000 loss
C €6,700 loss
D €1,500 gain

The solution is:

B
Relevant costs if built inhouse, €000:
Materials – 58.5
Labour – 32.8
Additional overheads – 17.2
Lost contribution – 28.5
Total – 137
Contract price of outside supplier – 110.0
Difference = Loss if done inhouse – 27.0

I can’t understand how this lost contribution of 28.5 was calculated.

Many thanks for your help.