Generally, expect cost to be (caster level × spell level × 10 gp) plus or minus a fee or discount based on a reaction roll, 2d6 + Level + CHA bonus. On a 2 or less, double the cost. On a 3 – 7, normal cost. On 8 – 11, 10% discount. On a 12, 50% discount.
For example, a 6th level cleric is asked to cast Cure Disease, a 3rd level spell. The base cost is 6 × 3 × 10 = 180 gp. The character asking for help is 2nd level and has a +1 CHA bonus. The reaction roll comes out to a natural 11 + 3 to make 15. The fee is reduced to 90 gp.