It depends on how good you are, how many mistakes you make and how long it will take you.
This is typically a thing where one would pay for the result, not the time invested. You charge for your excellency aka make the money from "what time it would take them" to "I am faster than they are".
At some point it is a question of "will it be cheaper to give to the intern". Also usually companies wanna post themselves to get the added exposure for their pages.
If you want to make this a regular deal, this will be mostly about process and setup.