Take the # of hours it takes you to do it and charge $50/hour. That's the price. Eventually you can charge $100/hour but that will require a bigger customer.

If the customer is small < $1M in gross sales per year - charge $50/hour
If the customer id medium < $1-5M / sales - charge $75
Over $5M - charge $100

The challenge you'll face is clearly defining the expectations and handoff so that you're not stuck doing stuff that you can charge for and always getting interrupted from past customers.