Well technically they aren't the only ones- even though you're 'using it anyway' the cost of heating, electricity, cleaning materials etc. should also count as overheads, although they may be negligible. Also, your 'salary' per hour should also be factored into the cost of providing the service - although the dogs are just sleeping, you still have to be present except for those 3 hours you're allowed out.
Finally, if you think about it from a business POV , you (and others in the industry) get zero benefits from publicizing your low overheads, and making it sound easy. Not only will you encourage competition - although I'd imagine the size of house needed is a barrier to entry - although someone else with a smaller place could board less dogs. You'll also get people complaining about how expensive you are, considering you profiteering, and all sorts of other things!
I'm not sure what your intention was at starting the thread - miffed at people thinking you earn 'pocket money', curious at the lack of understanding regarding how easy , stress-free and decently paid your job is. All of this only benefits you, so in your shoes, I'd actually be happy that they don't know. And not attempt to correct them...