Since any attempt to bring back a dead person interferes with Zespar, it requires dealing with that god. If a spell is cast by a cleric of another god, then what actually happens is that the cleric speaks to their god and then that god speaks to Zespar. The exact motivations of the god of the dead are a mystery to all mortals, but it is known that Zespar balances the souls that return: if the follower of one god returns, so will a follower of an opposing god. Needless to say, having to ask Zespar for a favor is something no god wishes and thus it will only be done for very few reasons, usually requiring the returnee to be of the faith (or very friendly to it), with a large fee on top of it (and often requiring service to prove the favor useful). The exact costs depend on the length of time the character was dead, the spell used to resurrect them, and the level of the character (higher level characters influence the world more, so Zespar is more reluctant to return them).
Arcane spells used to bring back a character follow the same sort of guidelines. The caster asks Zespar for the soul back, and Zespar will usually agree in return for some service. Thus, the cost for hiring an arcane caster is usually much higher than the clergy.
There is one last way to return: bargaining directly with Zespar. This is usually fruitless, but occasionally can happen. Those who return this way are bound to a task which usually takes years to complete, and most often in next to impossible. As well, the person is returned powerless (effectively level 1). Since most who return like this have many enemies, they are usually in bad straights.