I would expect few spell-casters to be using a pay-per-spell model. A salaried position would be what most would be on. Religious healers probably tend their religious community for free. Lay healers might work on an insurance model.
Not that it should be impossible to lure a spell-caster away from his regular work to do a bit on the side. But such arrangements are likely made on a case by case basis and the cost unlikely to be standardized except in two cases:
1) Widespread pay-per-cast spell use with a competitive market, in which case the cost will be fairly low. A spell caster who tries to charge those small fortunes for a single spell will be undercut by hungrier competitors.
2) Magic use has taken a guild structure and the guild regulates pricing. In this case, costs could be quite high, although government interference or a concern for public image may keep cost down, although nowhere near where they'd be in a competitive market.
Also, the type of spell and circumstances of casting are going to be more important than the level in pricing. Healing spells may be subsidized by a government that recognizes the value of a healthy population. Any sort of offensive magic is going to cost a great deal more because of the risks (not only in actual casting but future repercussions). Spells that deal in information may be cheaper if the information is also of some interest to the caster (or the caster is just a general busybody). If the spell-caster must travel to cast the desired spell, the hiring party must expect to pay for time lost and cover traveling expenses.
Strange outlanders with rumored wealth can expect to pay more than a townsman or rustic the spell-caster may well have known for decades. If the need for the spell is considerable (curse removal, healing, and the like) and the casting is done for mercenary purposes, the price is likely to depend principally on two things: how much the caster thinks the party has available with which to pay and how far they can be pushed before having one of those psychotic episodes for which "adventurers" are so well known.
On the other hand, if one has done a service for the local community, especially a small one, healing and minor spells may be done freely out of friendship. Or it might be done in trade for a future service. Trading in favors is an excellent approach for a spell-caster who hopes to have some control over his corner of the world, for whatever reason.
And all this assumes a fairly "advanced" society. In more "primitive" contexts, you'll be looking at some sort of barter or needing to negotiate with the chief to gain his favor rather than trying to hire the spell-caster.