I think one really needs to think more than just about the money when they donate sperm. They need to do a lot of self introspection and decide if knowing there are children out there, that are biologically yours is something you are comfortable with.
You may think that means nothing, but as an adoptee, who wants to know my biological history - I find it very plausible that these children may seek you out when they come of age. Would you be willing to have a relationship with them? Would you always wonder about how many children you may have out there? One could argue that my birth father was just a sperm donor, which for all intents and purposes he was - he got my birth mother pregnant and ran - but I still have a desire to know more about my genealogy, who he is/was, etc.
When you're young, all you think about is the money that you can make by donating sperm, when in fact, I think that once you mature, you may have regrets or questions that can't easily be answered (who are my biological children?).