While there are always exceptions, the greater the wealth, the greater the ability to pay. The first few dollars of a person's income almost always go for essentials. As you start to make more, there is more opportunity for discretionary items. That is one reason why, in my opinion, the law of tithing as preached by the LDS faith is simply wrong. Ordinary living expenses are not "increase." They are simply taking care of staying alive. So long as a person does not have a lavish lifestyle, I believe a proper tithe should generally be based upon increases in their net worth, not their income.
Some wealthy people have a tendency to become greedy and decide not to tithe, but in general, I think if reliable statistics were available, you would find more tithe payers among the more economically affluent than among the poor.