Hi Mark,
I give my clients an estimate based on the information they’ve given me and how many hours I think it will take to do the job. In my agreements, I clearly indicate that the price is based on X number of hours and that additional time is available at my hourly rate.
Communication with your client is key. You really have to be clear about what they want and how long it will take for you to do it. Don’t be intimidated if your quote is higher than what you’re used to. We’ve all been at a point where our quote was a scary number. It’s part of your success, enjoy it.