I've SBF'd for free many times. If you need to pay to SBF...you're looking in the wrong direction.
Cryptic, somewhat. Yeah, I know.
However, there are those who actually aren't looking to make money to help you get to where you need to be on your phone. It's becoming scarcer by the minute though, so be quick on your google skills.
EDIT: However, people who help you can advance their help by DONATIONS. I've got many free apps and I do donate to a few of them to send my appreciation and hope that they will still support their apps. You can either choose to donate or not. If you feel their time and effort aren't worth even a penny, then don't donate. I've been on unemployment and donated to help others. YMMV.