hehe, this is indeed a good question. Actually, some coppermine devs might be rich, some might be poor. However, we spend our time working on the project without getting paid, so the question about the devs' financial status just doesn't apply. It's a hobby (you might consider it some kind of "work for the community", even call it "charity work"). Not only the coppermine devs are doing this, but thousands of other volunteers - this is what you call OpenSource movement. OpenSource software can both be used by home users as well as commercial users. A well known example is the free operating system Linux: both volunteer (unpaid) developers as well as companies contribute to the development of various flavors of Linux and apps that are based on it. Some flavors of Linux come completely without charge, others charge for the package (they usually come with additional extras like a printed manual, premium support). There's a great variety of licences that OpenSource software is being released under. These licences determine how an OpenSource package may be used, modified and published. Coppermine is being released under the license "GNU GPL" that basically says that you're free to use and modify the software, you're even allowed to sell modified packages; however, you have to release all resulting work under GNU GPL again and make the source code of the app available to everyone.
The GNU GPL licence would allow the coppermine developers to accept donations, but we agreed that we wanted to stay financially independent - all of us have regular paying jobs, coppermine is just our "pet project". If the costs (e.g. for webhosting) might increase one day over a certain amount, we might change this policy and decide to accept donations to raise funds for maintaining the site.
The only thing we ask from users is that the "Powered by Coppermine" tag stays visible on your site. The reason is that we'd like to give others, who come across your site and like your gallery, the chance to get their own coppermine for use on their own site.
It's great to hear that coppermine users appreciate our work and are even ready to donate money - thanks for your readiness to pay for Coppermine.
Joachim