In the US, you don't have to pay disability lawyers upfront. They get paid a percentage of any back pay they get you. It's usually the smart move unless you have some undeniable physical disability. They know the right words to use to make sure you qualify.

For me it really worked out because my lawyer argued I was disabled much farther back than I thought I could. I ended up with much more back pay even though he got 30% of it.