It depends on the character level. At 1st level, a wyrmling dragon gland (which deals the same amount and type of damage as a Haan's Firespray ability) cost 76% of a character's total wealth. Additionally, it has a DC of 14, likely higher than the DC of the dragon gland (10 + half level + Con mod), and it has double the range (30 ft. vs 15 ft.). At first level, a single use of Firespray is more powerful than a single use of a wyrmling dragon gland.
Of course, as weapon and special ability damage goes up, Firespray becomes less useful. It has a fixed DC (whereas the dragon gland's DC will rise with level), which will make it useless at some point, regardless of it's range.
If I had to hazard a guess, I would say the turning point is somewhere between 4th and 6th level. Below that, buffing Firespray would be overcompensating, and above that, buffing would be necessary to keep it useful.
Now, exactly how overpowered is a matter of opinion. It could be such a small difference that it's a non-issue, or it could allow one player to dominate combat encounters at low levels. It depends on how it's played.