First off, this is not some self-help mumbo jumbo about self-esteem, but self-esteem and how you value your self and your time absolutely affects your overall worth. Now, for the sake of this piece I'll be focusing on determining if a page rate can work for you and if it doesn't what--if anything--you can do about it.
So, despite your parents protests, you've decided you want to letter comics. Let's assume you have a solid foundation on how to actually do this and you've proven to yourself you can do it well. You've put your work out there, submitted samples, and hey! someone has just offered you your first gig. Congrats!
The client has neglected to tell you what your page rate is, and being the smart individual you are, you ask them, "what's the rate on this?" They reply, and it's 10 dollars a page. It's probably not the answer you were hoping to get, and you respond asking if it's possible to get more. Turns out to be no. This is your first gig after all, and it's with a smaller--but reputable--publisher. Now you have to ask yourself if you're worth that? Are you worth more? Chances are you're worth more, but you decide you love comics. The project is a good fit for you, and you're just excited to get your first professional credit in a comic book. It really is a great feeling. Enjoy it, be proud of it.
You now have a baseline page rate. 10 bucks a page. There's probably 22 pages in the book. If you're decent at math that's 220 dollars. Factor in 30% for the tax man and you're going to take home 154 dollars to letter that book. If a client offers you less than 10 a page, walk away. Seriously. In my opinion this is the bare minimum page rate even a beginner letterer should take.
Now the gut-check begins. Can YOU realistically make that page rate work for YOUR life. You seriously need to take a look at your ability and speed. At this rate, speed will be your best asset. Not only to get more work, but to make the most efficient use of your time and page rate. If you can develop a workflow that moves you speedily through pages, while delivering GOOD work this will develop your overall ability as a letterer but earn you the ability to get more work and command a better page rate. Why? Because you're worth it, and you've proved to your client your worth it. Now, it may take you some time but if your persistent and consistent it will happen. (You have to ask of course.)
Let's say you're able to letter 4 pages per hour. Instead of looking at it as a page rate, you could change the way you look at the project to an hourly wage. Now, you're making 40 dollars/hour (don't forget the tax man!) that's not bad. Also keep in mind time beyond the initial page. Keep track of how much time you spend on communication with your client, on corrections, proofreading, re-writing, file delivery and then factor that into the rate. Is it still worth your time? How can you make it so that it is? I have no easy answer there, that's really up to you. But if you really want to be a letter, speed is the name of the game. Speed kills.