Here are a few tips beyond just doing a broad flight search.
Finding anywhere near this cheap of a connection seems to require no value for your time and a lot of flexibility but I did a search of several cities which you can find $1 bus tickets to. It'll also take a lot of searching.
There don't seem to be that cheap of direct flights from Boston, but you can look for nearby cities you can find $1 bus tickets to, like NYC, or you could look for flights to somewhere like Las Vegas and look for a $1 bus from there to LA. Here's some info on how to find those (full disclosure: I work for this site).
The other thing you can do is look for flights where Spirit charge their lowest fare and patch two together. You can save about $18 off of thier fares if you go to the airport to book, so currently their lowest anywhere seem to be around $16 at the airport. From there it's a big jump to their $100 transcontential fares. But you can patch two or more of these cheapest flights together somewhere in the middle of the country like Chicago.
For instance I found a flight from New York LGA to Chicago for $16 and the you can flight onward from Chicago to LAX for $36 (after booking at the airport discount), plus megabus is about $5 from Boston to New York and you have to stay the night at La Guardia airport. Total $57.