In my location, with a fairly large concentration of organic gardeners and an active PUSH for using natural fertilizers, I could easily get $4 to $5/50lbs bagged and delivered to a central location
HOWEVER, it's really really hard to get anyone to come to the farm to pick it up. We also have a couple of folks who advertise "free...you scoop...all you want" and they have a FEW people, but not a lot. One rabbitry (now defunct) had a guy who would come out every week and take home a trailer full of manure...he'd bag it up in town and go around his suburb selling bags.