There's another possibility that I didn't see here.
I usually help people hiring developers and when you get quotes that are that different with people on similar seniority levels it's usually because the requirements aren't specified enough.
It costs time and efford to come up with detailed-enough specification :)
I once wanted to take a project with BigCorp, my first offer was dead cheap since their requirements were consisting of two lines of text. So I gave them a feel of what I'd build for them within their tiny budget. 4 years later they still try to get somebody to build that thing for them, but specs are now 40 pages long and budget is still tiny. I dropped out 2 years ago (after providing n-th offer quoting 15x more than original quote), at the time I was already smelling a trap project, now I'm glad it went that way, I feel sorry I spent time preparing those offers.
I usually help people hiring developers and when you get quotes that are that different with people on similar seniority levels it's usually because the requirements aren't specified enough.