Doing low quality work is a great way to lower both your reputation and job satisfaction. And you better be 100% certain of each customer's lifetime value, otherwise you'll have a problem.
This resonates with me. When i was younger I used to get ignored a lot at certain kinds of store. Now I have more money I still avoid them on principal.
Anyway, if you want to serve low-margin customers, optimize your business against them, and be happy with those low margins, because you won't get any of the high margin ones.
Also, publicize your low-margin focus (tell up-front that you won't do non-billable services, and what service comes with what item), otherwise you'll be full of annoyed customers wanting reparations after a bad interaction.