have a look at this HHO gas is a genuine gas and can be burnt. Hydrogen Technologies have been using it in a test vehicle.. and on their site it warns about scam sites using the technology to run engines off HHO gas.
Have a look at
Hydrogen Technology Applications, Inc. for more info this looks like the real deal and even they say powering your car on part HHO gas (from electolysis of water) and normal petrol is some way off yet, let alone completley powering your car of HHO gas alone.
Overall cost needs investigating yeah might get more milage by burning HHO gas mixed with petrol but what is the cost of creating the HHO gas in the first place??