Definitely a main line issue, or at least down from the toilet. I assume this must be a ground level or basement toilet right? I think the next step is a longer snake. I've seen someone on here suggest a guy that will snake any drain for $100, but couldn't tell you who. Might do a quick search of other plumbing related threads. You may see if you can find a clean out further down the line and try from there before you call in anyone else.