Currently there is no "dragon bow" in game, however concept art has been shown for dragon ranged weapons. In the EoC there is a Dragon 2h crossbow, but that has not been released yet so no one knows how it will be obtained or how much it will cost. According to Jagex the dark bow was an alternative to the dragon bow, as the dark bow is the only bow to fire dragon arrows at this time.
It depends on the type of bow you want
They some bow ties cost more than 10.50 each.
To much....
50gp
they cost 200k.
at walmart it is 19.98
more than you can afford
a lot
They cost 19.95 because you get all six and you keep them.
A whole lot.
a lot
10m