Castles and Ruins gives the cost to employ casters on a daily basis, ranging from 7 cp/day at 1st level to 25 cp/day at 10th level (pg 40).
The rules in the Treasure Companion implies a salary of (PP spent per day)*(number of weeks) = price in gold. Reduce that to work on a PP basis (potentially on a single day) and it works out to 1.4 silver per PP. This is quite a bit higher than the Castles & Ruins costs, which could be due to the rarity of alchemists and the value of their products. But hiring someone to cast one spell will generally cost more per spell than giving them steady employment.
In any case, the price should depend on how hard it is to find casters and how willing they are to take jobs. So adjust as desired.