There probably is sufficient demand to pay for it, the issue is that there is no mechanism for orchestrating such funding while remaining uncompromised. If you split out the cost of salary for 1 or 2 people, you'd likely end up with individual citizens paying pennies to have people sit in and provide this information. If you look up the average population of a small city, where such an operation would be the least efficient, its about 50,000 to 100,000 people. That would pan out to maybe a dollar per year to cover the salary - I don't think many people would be opposed to that if they actually trusted it and the money was allocated efficiently.

However, there is no way to actually get that payment consistently. It would have to become a government subsidized operation in order to actually extract that payment at a consistent distribution, at which point a huge conflict of interest is introduced, and faith is lost in the independence of such individuals. As soon as this becomes a government apparatus, costs grow heavily to account for administrative overhead, and there becomes heavy incentives to provide more favorable coverage to political figures who are responsible for budgets.