It's important to recognize that it's easy for us, but not for people like your boyfriend's coworker. For her, even if the process is simple, it's a scary and stressful process. There is a very different "opportunity cost" for the two of you. And this is where trade comes into play.
There are a lot of economic factors which would go into calculating the optimal price, but in the real world you never actually pay/receive the optimal price anyways (even if you can accurately determine what it should be).
Consider this: even though you have told her that it's easy to do, she's unlikely to do it herself. She's going to take it to someone, and if not you it will likely be a more expensive professional. Look at what the professionals (like Best Buy or Microcenter) would charge for the same job. As long as you're coming in under that number, you're saving her money and she will be happy.
Also consider what your time is worth. As a rule of thumb, an independent contractor charges about twice as much per hour as they would expect to be paid for the same work as a regular employee. So if you currently work, calculate your hourly pay and double it to come up with your consulting rate.
Even for easy jobs, be wary of under-charging. Once you start undercharging people, you open yourself up to being taken advantage of. Also, by charging a higher (but fair) rate, you are demonstrating the level of skill and effort required, and people will be more appreciative of the work you do for them. If they find the price high, it will be more incentive for them to learn to do it themselves (and I always offer to teach people for no additional charge rather than just doing it for them.)
While your work may seem and even be easy for you, don't undervalue yourself or your skills. You want to be taken seriously and you want your work to be properly valued. Even though it's easy for you, it would be hard for other people to do (let alone do correctly) and it has a very big impact.
