I think you may be referring to "reg. fee" which actually means register or registrar fee, not regular fee.
This fee is how much it will cost you to register a domain at a given registrar.
For example, reg. fee at Dynadot is $7.99 for .com.
It can be hard to follow when someone says, "You can sell it for reg. fee" or something like that. But, you can assume it's somewhere between $6.50 USD and $10.00.