The fact that First Hampshire and Dorset is making a healthy profit, while First South West is making a significant loss is likely to make the calculations completely different.
It's actually around six months a year the open toppers were used for, not four.
First have broken cover about the removal of the open toppers:
This year we will be removing open top buses from our network in Cornwall and instead replacing them with covered double deckers. This means the buses will be able to run in all weathers, as well as continuing to serve the local communities who also use them in Cornwall.
www.firstbus.co.uk
Certainly, Cornwall tourism did have a poor year last year and a lot of businesses suffered as a result. Whether that is a trend or a blip remains to be seen.