This is a common thought process. Most freelancers work for too cheap. It’s a common problem. What is worse, the moment you start giving your services for cheap the consumer begins to feel, you are cheap because you provide a service that is lower quality than the other person who service is expensive. There starts the death spiral!
Moral of the story. Always offer a competitive price for your services and never offer a free service.
Reeading this reminded me of something that happened here a few weeks ago, basically these guys we called to come clean the AC's of the house said it would cost a certain amount. Since it seemed quite like a low amount we went ahead and rounded it up to give them a tip which they were grateful for.
The funny thing is after a few more months we call the same guys again and either they remembered us or had just happened to increase the price, they quoted us for the amount we had rounded it up to when being generous last time.
Guess no tip needed anymore. :D