I've a friend who has run his own small IT business for a bit over a decade. When I talk to him and he mentions how much he charges, I always tell him it is far too low. He sees it as being easy for him, so he doesn't think he should charge much. I've tried to explain to him that when the plumber comes over, you're not loading that guy down with quantum physics work... he knows how to unclog your drain or run a new water line. That's easy for him. He charges a high amount because his services are valuable. And I, for one, am happy to pay that plumber the high amount. It saves me having to invest far larger amounts in learning and tooling up to do it myself.

I've worked alongside him a few times on projects that his clients had which required coding work alongside the hardware and sysadmin stuff, and each time I've had to badger him into charging double or more than what he wanted to charge. And of course the customers paid for it, because it was still a good deal and I could show them a conservative estimate that said the system would pay for itself in savings in under 2 years. When freelancing, those are my favorite contracts. Where you can show to the customer up-front that you will be saving them money in the long run. It's always much easier to sell them at that point, and I think the amount its going to save is a pretty good proxy for the value of the work.