I was pricing Kerala house boats not to long ago. The kettuvallam look rather majestic in the pictures, like something you might see in an Indiana Jones movie. Renting them generally costs more starting in November, and lasts 3 or 4 months. They can be a little costly, depending on how many bedrooms and bathrooms you want, as well as other services. From my calculations I think a good but affordable one will come out to be about $200 USD a night, maybe a little more. I saw prices that were as low as $100 (but no pictures) and as high as $1000 a night, with an average around $300 or so.