I'm more concerned about my current & future business & what it generates while I own it. Neither me nor thee will change the opinion of what window cleaning businesses are worth! Why worry about it. As said, it's only worth what someone will pay for it, no buyer- worthless. Work your bollox off, have a huge turnover etc. it's still a "window cleaning business" which will probably be viewed differently to other businesses, especially by someone on the outside of the industry.
I think my house is worth 500k, unfortunately, no one else shares my opinion. :
BUT, good luck to the guy who's selling it.