Hmmm. Bat/bird guano as a fertilizer really is an artifact of the industrial age. Human nightsoil, manure from livestock, seaweed, composted plant material are all much cheaper and easier to obtain for the average peasant, who doesn't have the cash to spend on something mined underground.
Plus, if you have to hire adventurers to gather the stuff (as opposed to hiring commoner laborers), it's got to be too expensive a commodity to be useful as a fertilizer. I'd go with the magical/alchemical side--it is a spell component (fireball, I think?) after all. Pay them 100-200 gp a sack, or thereabouts--figuring that 1st level characters are going to be able to carry one sackful on each trip to the caves. They'll have to face some encounters each time, and you can gear the earnings from the enterprise to wealth by level requirements, factoring in any other treasure you hand out as "monster possessions."
In short, you should not let them dig more than a few sackfuls of the stuff per trip to the caves (unless their job is to protect laborers, in which case you can pay them a fixed wage and let some merchant handle the business end of things), and you should figure the sale price of the commodity based on what's needed to support your wealth-by-level progression, rather than relying on a random figure determined by its theoretical value in relation to other commodities. As DM, you can fudge the background economics to set the price where it needs to be to serve as adventure hook and treasure. You can even change the price on short notice. The king wants to keep wizards from casting fireball or a wealthy merchant is sponsoring some alchemical project, so he buys up all the bat guano on the market, driving the price up. Or someone sells off his stocks of his stuff, or the wizard-war in the neighboring realm ends, increasing supply relative to demand, driving the price down.
Plus, if you have to hire adventurers to gather the stuff (as opposed to hiring commoner laborers), it's got to be too expensive a commodity to be useful as a fertilizer. I'd go with the magical/alchemical side--it is a spell component (fireball, I think?) after all. Pay them 100-200 gp a sack, or thereabouts--figuring that 1st level characters are going to be able to carry one sackful on each trip to the caves. They'll have to face some encounters each time, and you can gear the earnings from the enterprise to wealth by level requirements, factoring in any other treasure you hand out as "monster possessions."
In short, you should not let them dig more than a few sackfuls of the stuff per trip to the caves (unless their job is to protect laborers, in which case you can pay them a fixed wage and let some merchant handle the business end of things), and you should figure the sale price of the commodity based on what's needed to support your wealth-by-level progression, rather than relying on a random figure determined by its theoretical value in relation to other commodities. As DM, you can fudge the background economics to set the price where it needs to be to serve as adventure hook and treasure. You can even change the price on short notice. The king wants to keep wizards from casting fireball or a wealthy merchant is sponsoring some alchemical project, so he buys up all the bat guano on the market, driving the price up. Or someone sells off his stocks of his stuff, or the wizard-war in the neighboring realm ends, increasing supply relative to demand, driving the price down.