In one study, the amount of salicin after 1 and 2 year growth in autumn and spring ranged from 0.08 - 12.6%.
Source:
Willow bark | University of Maryland Medical Center http://umm.edu/health/medical/altmed/herb/willow-bark#ixzz39e1Laclv
I'll play with some more consistent numbers, in a hypothetical situation.
Lets say within an area, most willow trees of a specific species produce 80-120 mg / g of dried bark. So instead of varying from 0.08%-12.6% we've changed it to 8-12%. I had to look up a standard pill - 325 mg. So 3 g of bark will get you about a typical pill. There is also a compound that has harmful effects, but you need 3.5 - 4 g of the bark before getting quite sick. Some of the trees fall outside of the range, and 3 g of the bark will cause them to be ill due to the secondary compound. Some are also great at making ASA, whereas some produce very little, so the 80-120 mg /g only occurs 99% of the time, and 1% of the time someone either gets no benefit from ASA, or gets way too much.
^ That's where non-purified, non-standardized basic natural products. You need 3g of bark to get a basic dose, and you can't take as much as normal, due to another compound. There's also the risk of getting sick from the occasional batch that contains more of the compound, and sometimes overdosing or underdosing is an issue. Mixing up the bark from numerous trees can help prevent some of the issues, but if a company starts to grow their own trees, that originate from only a few 'parent' trees, the might end up with a large percent that are outlayers - producing lots or little ASA, or having a large amount of the secondary compound.
Now take this out of a company and into the hands of the public. Are they actually starting with a tree that's the right species? Can we trust them to dry the bark enough? There are enough problems with people taking medications properly, now we are going to give them information about mg/g of dried bark and trust them to weigh out a dose? What about when we throw in other natural products to the mix. People mix up labelled meds all the time, I think baggies of dried bark & herbs would be worse.
Rather than putting in the money & effort into studies like these, I would rather we find new treatments that we don't have already. Personally, rather than going out and finding willow bark, I would rather just buy a pill.