If you can’t afford to file an appeal with the BIA, you should ask for a fee waiver. The fee waiver is on form EOIR-26A. The practice manual says the form can be filed on tan-colored paper, but white paper is just fine. The form requires you to swear under penalty of perjury that you are “unable to pay the fee,” so you should only file it if really can’t afford the appeal.
If you ask for a waiver, it is almost an absolutely certainty that the Board will grant your request. I have read thousands of unpublished BIA decisions in the last ten years and have never seen the BIA deny a fee waiver request (as long as it’s made using the correct form and with accurate information).
The Board does deny appeals and motions when no fee or fee waiver request was submitted. Usually such appeals or motions are rejected outright before the Board actually decides their merits. But from everything I can find, the Board tends to grant fee waiver requests as a matter of course.
For that reason, your lack of financial resources should never be a reason not to file an appeal.