Since you only have groupings and not exact figures for each one has to make assumptions.
In your shoes I'd set the payment for the group to be the middle value in the range.
i.e. 15% pay 1500
After that I'd select a value for the total number of people and then see what the average proves to be.
e.g.
55 * 500 = A
15 * 1500 = B
13 * 2500 = C
12 * 3500 = D
5 * 7000 = E (This is the most dodgy assumtion but in the absence of accurate figures ...)
A = B + C + D + E = Z is the total
Z / 100 = average
I've not tried it, this is off the top of my head, but I think it's how I'd tackle it.