S$250 is daylight robbery, just get the chemical yourself and you can it yourself, no need depend on these contractors
You can get the chemical called HCL89.
I usually get it from http://hcl89.byethost10.com or http://hcl89.atwebpages.com
What it does is that it cleans your wall tiles/glass and also helps to create a protection for it so that always remains new.
Hope this helps you out