I'm presuming that this is a 3.* setting. Translate as needed.
Seen this request.
The best answer would be to have someone post a bounty on various specific monster parts. They need to be very fresh, or expertly preserved.
I mean, we've probably all seen low level PCs, desperate for more cash, trying to skin various animals they fought so they can sell the pelts, right? (The answer on that is to require ranks in Profession - Trapper/Trader, which says exactly how much they can expect to get from their collection of pelts.)
If the PC wants to gather some for his own use, that's fine. It will save him/her some cash in the cost of such things. Of course, they need to be very fresh and/or expertly preserved, *without the use of magic*, since that would taint them for use as parts for potions or other magical crafting.
So how do they "expertly preserve" parts? Sounds like they need some ranks in Profession - Somethingorother (It's a Name you Skill kind of thing). Not that different than calling for Profession - Herbalist for someone to gather herbs and plant parts for alchemy or to sell.
Profession skills tell, by the book, how much of a living a person can make, if they dedicate themselves to that full time. Cut the profit by half if it's being done incidental to adventuring. (Survival to feed yourself cuts travel speed by half, since you have to spend half the day gathering food, which gives us a baseline.) Cut their profit further based on how dedicated they are.
In other words, when the caster wants to make his Portable Hole, he needs to find Phase Spider webs kinda like now. Two week old stuff has dried out a bit too much. Unless, of course, he/she knows how to prepare and preserve them. How much can they prepare and preserve? Count it in GP by checking their ranks in the appropriate Profession.
So rather than having this as an open opportunity for a crafter to power game or greedy PCs to pad their wealth, it becomes another treasure type, for those skilled enough to gather it, and an ongoing plot hook in the campaign.
I mean, what else is Profession used for?
