That is, the art itself has no inherent value. The actual value of a piece of art is whatever you can convince someone to pay for it.
One thing that can help is having receipts of what you've sold an artists work for in the past, since you can use that to inflate that amount a current piece will sell for.
To put it another way, how good of an artist you are is secondary to how good of a salesman you are (or how good of one you hire on your behalf) when it comes to making a living.