Some (large) conferences have a need for a large number of "helpers", called "student volunteers". In exchange for some number of hours of work (registration table, making sure speakers have water, lots of things) they get full attendance for free. I don't know (but doubt) if there is an additional stipend.
Many, perhaps most, of the student volunteers may come from one or more local universities, but I don't think that is required.
Such conferences may have a "call" for student volunteers on their web page.
Travel and accommodation may still be the biggest expense, but it may be that some conferences can help, especially with the latter if you are willing to share a room with a stranger.