As much as you can as glass roofs are a pain in the ar5e, Ian your a salesman, sell it to them mate, i reckon your not far off with your price though, say it an see what they say, give them the £180 and then keep stum, if they dont say anything then dont you either, if they hum an har over it then say agh well as your a regular then ill knock some money off and then reduce it to £160.
Is the access for the other side easy? looks like its off a flat roof to me