The price for using a math library developed by Foobar Corporation is $10,000 plus 1% of the sale price of your product, or whatever you negotiate with them.

The price for using a GPL’d math library is that you must license your source code under the GPL as well.

The price for using an LGPL’d math library is distributing the source for the library along with your product. If you modify the library itself, then that falls back to the GPL.

In no case are you getting something for nothing. It’s just that the cost of using GPL’d software isn’t usually in dollars, but in contributions of more software.