What you describe appears to be perfectly legal.
The GPL allows any licensee (anyone who bought it) to sub-license it to others for any price they want or even for free when they feel generous. Those other people are now also licensees who can also sell or gift the software to others. So when someone offers a GPL software for $100, and you want to pay less, you can seek out any of the people who already bought it and ask them if you can buy it from them for less. This is perfectly legal and intended. The only downside is that you won't get the support service offered by the original creator, because that's a completely independent contract not covered by the GPL. But when you need help with the software, you can make a support contract with anyone else you consider qualified.
However, that only applies to people who actually did receive the program under the conditions of the GPL, either directly or through any number of sub-sub-sub-licensees. Should one steal a copy of a GPL program illegally (through hacking, for example), they are not licensees, which means they do not have the right to sub-license it, and are violating the copyright of the original author when they do.
<advertising>When you have more questions about how one can monetize GPL software, you might want to commit to the new proposal Open Source Stackexchange. It still needs people to commit to it so it can go into beta phase.