"Mailorder Spell Service" (5e)

The correct answer is whatever amount you feel like charging. However, there's a lot of factors that can inform that feeling. Consider the following questions as you try to arrive at an exact figure:

How common is magic to begin with? Can anyone go to wizard school and learn to make a living selling basic spellcasting services? How much wealth do you want the PCs to have after buying such services? Are the prices for spells going to be negotiable? Can the PCs barter instead? Do they need wealth to purchase other important items still? This last one is important; plate armor is the biggest "normal" expense, but once purchased, there's not a whole lot of other useful adventuring gear to be bought with whatever wealth they do acquire. Unless, of course, you allow for purchasing magic items.

If you're looking for more concrete answers, you can take a cue from Adventurer's League table of spellcasting services.

Cure wounds (1st level) 10 gp
Identify 20 gp
Lesser restoration 40 gp
Prayer of healing (2nd level) 40 gp
Remove curse 90 gp
Speak with dead 90 gp
Divination 210 gp
Greater restoration 450 gp
Raise dead 1,250 gp

From these figures, you can see that the cost is basically Spell Level^2 x 10 + material component cost (divided by 10 for reusable components with a cost) + Z, where Z is an (apparently) arbitrary amount based on the projected treasure accumulation of players in Adventurer's League. It should be noted that Raise Dead is a major outlier, and for good reason. In AL, players get a free raise dead from their faction until they hit level 5. I look at the cost on this table as redistributing the cost of raise dead at lower levels to higher levels.