£25 is for 1 to 10, you are not going to charge £25 per unit if you have 200+ to clean
if we had say 30 to clean then it would be £15 per unit
this is one person taking on so many and if you have an employee trained to do it then he/she can take some on. Regular work you dont need to rip them off.
This is like FOX said, monitors, wires, mouse, we clean around the desk and sanities the air in our service.
Commercial, like server rooms, then you would be charging some money.
We have done this 14 years and moved on to work stations.
And for the record we have 3 chaps working at the same time so based on 200 £2.66 per work station this brings in £31.92 per hour 
£9.92 profit per hour
(this is £9.92 after products by the way), wages £21ph based on 3 chaps, £10.92 left over and we buy and sell products so our product cost for comp valeting is £1ph 50 man hours to clean 200 comps so per clean turnover £532 profit per clean £132
we have 22 clients at the min so works out well