If you're being paid to do work (you're not working for free) in this situation, you can conclude that either a) you actually are providing roughly $YOUR_BILLINGS of value to your client or b) your client is a moron and is paying for zero value. You may not be able to directly quantify the value you're providing, but your client certainly is aware of the value you're bringing. The point is to try to more accurately quantify your value and capture more of it.
> Measuring value in software development is difficult.
Again, your client has a ballpark of what value they expect you to deliver. Measuring the value is difficult for the contractor. This is a great area for discussion with the client.
Summary: if you're being paid, your client perceives that you are providing business value.