Cost for bamboo housing?

Cost of a untreated bamboo house with thatch roof (around 200 square feet) = Tk 5,000
Cost of a treated bamboo house of same dimension = Tk 6,250
Cost increase due to preservative treatment = Tk 1,250
Cost of treatment = 25 % of the cost of untreated house
 Service life of conventional untreated bamboo house = 7 years
Minimum expected service life of treated bamboo house = 15 years
That is, 25% increase in cost results in 100% increase in lifespan. 3.6

Source- http://www.adpc.net/igo/category/ID189/doc/2013-p74Wob-ADPC-handbook_complete-b.pdf