I definitely recognize the point of few career prospects making potential applicants hesitant. That's a point I hadn't considered before.
As to the price point, I'm not convinced. Raising the potential salaries for these positions would attract people, obviously, but the price point would attract them away from their current jobs, leaving a vacancy at another company. If this is the situation, raising the salary doesn't suddenly generate new engineers with the necessary skills. The problem is the existing engineers with the necessary skills are already hired and fulfilled leaving only the option of headhunting.
At a more practical level, yes, if we hired these positions, our capabilities would be extended greatly. Hell, hiring one would be enough. However, there is not enough value in that new hire to justify raising the salary above it's current position.
And lastly, no one has been offered the position and then rejected it because of the pay. The last 3 offers given out were immediately accepted. We just haven't found anyone worth giving an offer for these remaining 4.