"My work is not free."
Translation:Mia laboro ne senpagas.
Free as in Beer: sen + pagas (without + pay).
Free as in liberty: liberas (some people would say "Stallmanas" for software, but don't believe them).
Senpaga is an adjective that describes something you acquire without paying for, and senpagi is the action of doing so.
I take it you can't mean the same thing in a a valid sentence by saying "Mia laboro pagas" or "Mia laboro estas paga".
Ne and sen together seems similar to a double negative.
Another valid translation for this sentence is "Mia laboro ne senkostas." I prefer it because it also works when you remove the double negative - "Mia laboro kostas."