Imo it is very low, below the 10 M mark. I mean, during Sabaody we got the supernovas who had a bounty above the 100M, there were just a bit more than 10 in the whole arcipelago and it should have in total some thousands of pirates at least. The supernovas put up a fight vs admirals and marines but so did their subordinates who are below the 100 M, so Sanji, the poison guy from Kidd crew, Bepo etc. I haven't seen other pirates even try to fight, or put up a fight, vs the marines. Imo the average lvl are the 2 fodders who tried to kill Kizaru with a rifle. They should be the Sabaody fodder and so have great numbers who bring the average bounty very close to their bounty level.
Let's say you have Luffy worth 300 M and 99 of these guys worth 1 M each:
(300 + 1*99)/100 = 3.99 M average