We actually started building such a product for India where the problem complexity is much smaller (only about 20 prominent airports, majority travel by train where prices are static, only availability varies with time).

Availability of data APIs turned out to be a big hurdle for a bootstrapped startup. The market is still up for grabs. Here is another attempt by an Indian startup:

http://www.90di.com/travel/#form_based_search_home_page