I won't throw in specific numbers, but I have clients that range from $10/mo - $10,000/mo. And the company I work for has clients ranging from $100/mo - over $90k/mo.
Its a mix bag. As long as your ROI is good, adwords works great. But if your selling something at lets say $50.00 and it costs you $40.00 to advertise before you get a customer, adwords might not be for you. Unless you are happy with $5.00, and/or if you are selling in bulk.
Now I do have a friend who only makes $0.03 - $0.75. But he is doing a referral service. He spends on average $0.35 per keyword which he gets between $0.38 - $1.00(or so) per traffic. So its not a lot, but considering he gets roughly 12,000 clicks per day that is a nice little $480.00 profit without doing much of anything.