Sure. This is, of course, a very controversial area, particularly with the advent of profit-maximizing investors investing in microfinance. There have been some spectacular examples in India and in Mexico of investors making a great deal of money when an IPO is issued for a microfinance institution.
The issue for interest rates is that it is more expensive to reach the poor. Quite often you have to take the bank to them, which means you equip a credit agent with a motorcycle and off they go into the hinterland, or even if it's in an urban area, it requires leaving the bank and coming to the community. That, by itself, requires special training, special support, transportation, and so on, so it's more expensive, plus the economies of scale are not so favourable in that it costs as much to make a thousand dollar loan as it does to make a hundred dollar loan. If you have a whole bunch of hundred dollar loans, that's problematic from the point of view of making enough money to cover your costs.
Those are the basics, but there's also a good deal of difficulty in really understanding what microfinance institutions actually charge, what the true effective interest rate is. Even the providers of such loans often don't really understand the true APR of their loans. There's a movement afoot to really become much more transparent about this so that the poor, when they have alternative sources of financing, can compare them more on an apples-to-apples, and oranges-to-oranges basis.
You asked about the average. It depends on the country. Mexico is very high,so 80% APR would probably be typical. In Bangladesh it would be maybe 20% to 30% APR.