Access to the far side looks awkward. From what I can envisage theres a wall there so access at the very least will be off a ladder, which you'll want footed.
Price is dependant on how much you want/need to do it and who it is for (regular customer anyway?)
It looks very similiar to a greenhouse we do for a regular customer occasionally.
Being realistic you could knock that out in 2 hours doing a proper job, which according to the generalised and over-egged CIU hourly rate of £30 comes out at £60. However, allowing for a 50% margin of error you'd have to increase the price to £90.00.
However considering its a nice piece of architecture and is in the grounds of a wealthy individuals estate (this means it attracts the higher rate of 'wealth tax'), it looks nice, and ideally will continue to look nice, that it has a south facing aspect, is green, made of glass and isnt full of illegal immigrants I'd have to put a price of £160.00 in.