Start small, test the waters and get your name out there. Quality and customer service is key. I do quite a few one off prototype prints for various clients but it's pretty small dollars. Let's face it, if someone calls and wants a fairly simple prototype printed that takes an hour or so you can't charge them hundreds of dollars. It's maybe a $30 - $50 job. Where you can really make the money is 3D design and scanning. For those types of services you can charge considerably more money. When you can offer full service such as 3D printing, design, scanning and reverse engineering, that's a pretty good model to start with. I've found there's a need even at a basic level. I'm no engineer and certainly won't get calls from auto manufactures but lots of folks have much simpler needs and are willing to pay for it.