Having recently acquired Hedge Magic (4th ed) thanks to the clearance bundle, I am intrigued with the idea of introducing my troupe to a Spirit Master. I am having trouble coming up with what the hedgie might offer the summoned spirits in exchange for favors. It seems that spirits of sickness might want easy victims to infect, and ghosts would like a good burial place or some method of remembrance/honor. What else could motivate these spirits to do a favor? What kinds of things would motivate an elemental spirit?
Any brainstorming help is greatly appreciated!
More of their corresponding element. Or less of the opposing element.
Ghosts often tend to stay ghosts because they have an obsession of some kind: revenge on their killer, desire to protect their loved ones, etc. Anything that furthers their obsession is a way to get their help -- but be careful that if the obsession is completely fulfilled (say, the ghost gets his vengeance in full) the ghost may find peace and pass away (great for the ghost, not so great for the spirit master).
Realms of Power: Magic has some guidelines about Airy spirits in Chapter 7.
For example:
a faunus (spirit of an individual animal) would be interested in anything to do with that animal, likewise a flora (spirit of an individual plant) would be interested in little more than its tree or grove.
Earth spirits are interested in hardness, rigidity, and permanence; Water spirits like coagulation and mixing; Air spirits like flexibility and change; Fire spirits promote separation and identity.
Imagines (spirits of sensation) would be obsessed with the sight, sound, smell etc. that they represent. They might want more of the colour red, for example; or more rotting corpses to smell.
Eidolons (spirits of emotion) are interested in making an individual feel that emotion more strongly, or more frequently. They are usually obsessed with a specific person's emotions; so it would be the Spirit of Marcus's Greed rather than the Spirit of Greed in general. This spirit would concern itself with promoting and satisfying Marcus's avarice, to the exclusion of all else and regardless of the consequences to others.
The lugra (spirits of hardship) want to promote their individual brand of misery.
The desires of ghosts will depend very much on the type of ghost and the degree to which it remembers its life and realises it is dead. Magical ghosts remain tied to the world due to an overwhelming emotion or need; and they would probably want this to be resolved (whether they realise it or not).
Hope this helps
Mark
Spirits want spiritual stuff. Keep it abstract, keep it artistic, philosophical or ephemeral, avoid concrete items of value (except as sacrificed for some abstract and valueless reason). Keep it atypical for anything "human".
Also, look up the "grateful dead" tradition. Ghosts want what they miss from their mortal life.
While I like the suggestions above, I'd also suggest that at least some spirits don't actually want anything and only demand payment to deter magicians from harrassing them with summons - I think this may have been suggested under Ars Fabulosa actually...
That is a rather good idea. Give the ghost/spirit a folk ken or other suitable skill roll to find out what the summoner treasures most, and demand that in huge amounts. To be left alone later 
Helpful responses. Thanks!
They'll probably want that, true enough.
Most spirits want vis, too. They don't really care what type, though if it's particularly appropriate to them (like giving Ignem vis to a fire spirit) they may value it more highly. You might look at the rules for Hermetic Sahirs in Societates, which puts all the bargaining between a magus and spirit into a single Sihr Supernatural Abilty.