It seems that there is a few factors involved here. 100 gold worth or most common things seems like you would get a lot of that, but the more rare the ingredient the less you would get, thus a greater DC to find. Also, the fact that some things only grow in certain climates and you would never find them if yopu were looking in the wrong one. One last thing to consider is that some of these things you might be foraging for are animal parts, such as pixie wings and eye of newt.
As a DM, I might let you forage for 1/2 of the cost, and maybe 3/4 if you wish to track a pixie or whatever. The base time for the 1/2 would be based on a skill check. Survival DC15 takes 8 hours, DC 20- 4 hours, and 22 takes only 2 hours. If you wanted to use the Alchemy skill I would add 4 or 5 to the DC's above.