All builders cleans are differant.
Some get cleaning companies in to do the builders clean but not the windows, Some get a window cleaner seperate.
Others wants someone who can take care of the whole builders cleans including windows.
I do a lot of builders cleans and i get a freind who has his own domestic cleaning firm to do the insides and i do the windows.
Or he gets builders cleans and gets me to do the windows.
We pass work between us so it works well for both of us.
As for going rate, There is no going rate each site/Plot is differant so you do need to look at the job to work out how long it will take.
Take into consideration.
Is there tape on the frames
Film on the glass.
Cement on the glass and frame.
Paint on glass/Frames
Glue on Glass/Frames
Sealant on Glass/Frames
Builders cleans can be really hard going but then again you can get some fairly easy ones.
You need a CIS card if you are cleaning the insides that includes inside windows.
Builders are always slow at paying so if cash flow is tight builders cleans are not good to do.
Make sure your insurance will cover you for builders cleans and also make sure you have all risks insurance as its all to easy to scratch the glass if you have never done builders cleans.
Paul