S'mon said:
I think so (need to check DMG but off-the-cuff sounds right) - for that price (35,000gp) you'd need a 12th level cleric with 'Sending' and the 'Forge Ring' feat. You know, hm, 1 12th level cleric (Tarquin), I expect he has Sending but I doubt he has Forge Ring though.
Unless you're game world is different, you don't have to cast it at 12th level, even if you need to be 12th level for the feat (you can cast it at minimum level you would need to be to cast the spell). Also, I'm pretty sure you can use someone else who has Forge Ring and you (your cleric) supply the spell.
Randomling, my idea would be to make "cells" of spies. You have a leader that is in communication with the local members of his cell and only the leader reports to you. This makes things like Animal Messenger more workable if it is only being used in a small area (the area of the cell) and then you have dedicated messengers (and eventually magic items) to communicate between a cell's leader and yourself. Each cell should probably consist of a few Rangers to supply the Animal Messengers as well as keep an eye on the surrounding wilderness, a Bard or two to work rumors in more respectable circles in a city and then several Rogues to do the dirty work of actually bribing and spying and, if neccassary, assasination in the cities. You may also want a few extra Ranger, Fighter/Rangers or Barbarian/Rangers to act as an emergancy strike force if an emergancy come up and you need to extract a spy or harry some forming or moving troops or their supply line.
For money you can always have one of the cell members sell off secrets you don't need. Also, with all this information haveing a larger "cell" of merchants to do trade in the area, they would know what to bring where. If needed you can even use a cell to "adjust" supply and demand in the area in your favor (say your merchants get a lot of good wine in City A, send them towards City B, while City B's cell is covertly damaging local vinyards and causing the cities current wine reserves to meet with "accidents"). You would have to be careful with this though as it may get suspicious if it happens too much.
An excellent option here is that every cell could function independantly, with most members thinking they are just part of some shady guild. This is especially good if your merchants largley belive they are independant to try and create denyability between them and your "economic adjustments". Remember, the actual leader of the cell doesn't have to be the person who the cells members think is the leader.
You could even use the merchants (or a subset of them) to sell services as "bonded couriers" or some such, delivering messages for a fee. This way they won't know they are being used to help filter information between you and your cells. I wouldn't rely on the merchants as your sole means of communications though, it's too slow and not incredibly secure for such things. Rangers on horses do a fair job, especially if they have drop points to contact the cells. Oh, and remember to make some kind of code system for all your messages, "just in case". If you can design a good mundane communication line, you can make the Sending magic items even 1/week to reduce the cost more and only use them in case of emergancies.