I'm a 20 year old student studying Graphic Design. I occasionally get clients who would like me to completely redo their website such as designing a custom theme, adding plugins and doing it all with wordpress.
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.