I'm sorry, I haven't taken the time to read through everyone's posts, but here is my experience with trimming;
I've owned my mare since she was one, and she's had the same farrier since. I really love our farrier, he is firm, yet understanding with the horses, makes a very balanced hoof, and finishes his job by always making the hoof look very tidy. Not to mention, he is the go-to guy for one of the best local vets, so you know he has to be decent!
When she was two, I really started getting interested in alternatives to shoeing. As I was just starting to lead her on the trails, I noticed that her feet were chipping little bits at a time; hence, I googled trimming and maintenance between trims. I studied hoof angles, frogs, and whatever else you can imagine about horse hooves. Then I brought up the idea of rounding the edges to our farrier, and he really recommended that I take a rasp between trims to help keep the edges from chipping.
For about a year, I did this routinely. Still gobbling up information meanwhile, and I finally decided that I wanted to try trimming her feet. Terrified, I took the rasp and 'had at it'. Well, it wasn't all that bad, the farrier was out soon after to help critique me. For several months we went back and forth, me doing a trim and them him. As my comfort grew I started making the time in between his visits longer and longer. Now, 4yrs later, I plan to have him out 1-2times this year.
He has been very supportive and seems to think that it's wonderful that I trim my mare's own feet. Of course, when he does come out nowadays, he gets a bit of a tip or a hat, just something as a thanks, because I now he didn't have to be that AWESOME about the whole thing.
I now enjoy taking care of my horses' feet. I know how she moves, and I genuinely care about her. To keep them short and in good condition, I trim them about every three weeks. And, to this day I am always learning. My mom must think I have a hoof fetish, because I always have screens of hoof info and pictures on my computer!
Long story short, I do not think it is something you can just jump into; however, talk to you farrier and ask if he could show you how to do little touch ups in between. If you're serious about learning, then maybe a year or two down the road you will be trimming your horses own feet. If it is the money concern though, I do not recommend it. you have to be invested and actually want to help you're horses, not just try to save a buck!