I use the wonderful NiceHotel theme from quitenicestuff and FreetoBook.com as my online room booking engine.
My objective is to allow customers to check availability, pricing and to book a room directly on my domain: http://www.binghamtownhouse.com
Freetobook has some good booking widgets and a fairly open framework. I have asked Freetobook and they are also happy to support me having their booking widget embedded or served through my own domain rather than taking customers off to their own website to complete the booking.
Ideally, I would like to use the current NiceHotel design for the booking query and have customers visit a full page with 10 day view and rates based on their previous selection.
Please reply to this post if you are both familiar with the NiceHotel theme and are confident that you can this work in a reasonable budget.