How much potential? Qualified for sincerity, timeline and budget? Sounds like they just want to pay the minimum to get the minimum and that is the start of problems for YOU.
The client is not always right. The cool thing about consulting over designing is that you can come in and tell them and show them why ideas are good or bad (or could go either way) - charge for the 'lesson', and walk away paid. Then they can do whatever.
Many mechanics have signs posted to the effect that there is one price for doing the job. A higher price if the customer watches, and a higher price yet it the customer helps. A generally sound business model.
I charge 80 Euros per hour for writing content - I assume you calculated a set of hours for copywriting in your original quote? In that case just substract the amount. If not, split your quotes into different aspects of the project next time.