I am not sure it should be over 1 dose. Lets do a comparison.
The Dust of disappearance when sprinkled carfully lasts 1d10 +10 minutes. So, that is as if cast by an individual of 11th to 20th level. As the spell improved invisibility lasts 1 min/level.
A Single use, activated Misc magic item of Improved invisibility cast at 11th would cost:
11 caster x 50 gps x 4 th level spell = 2,200 gps
A single use, activated Misc Magic item of Improved invisibility cast at 20th would cost:
20 caster x 50 gps x 4th level spell = 4,000 gps
This would put it in line with any other single use, activated item cast of that value. The caster level listed is the listed spell level when you can get improved invisibility, but it is obvious the item doesn't cast it at the lowest level. 3,500 gps is about the average level at which it will cast it. If used correctly.