The answer will vary from employer to employer and depending upon what type of window cleaning they do.
Overheads are a big factor in any job.
Often employees will see that they are doing a job that is priced at say £100, however they may only be getting a small percentage of that and they think the rest is all profit for the employer, this is never ever the case.
Some jobs will have a good margin in them, others will literally cover costs only.
A window cleaner with a couple of lads working for him off ladders will generally have a good profit margin but a reasonably low turnover, these guys may pay out as much as 60% of turnover in wages. However some of the big boys out there may have 100 guys working for them and work to a much lower margin but have a turnover into the £000,000's, there wages costs could be as low as 10-15% of turnover