Let's breakdown the costs involved.
Each marketplace piece will run you about a dollar (with complex pieces or detailed bodies ranging from 3 to 5) and It's reasonable to say that a character will be made of around 5-8 pieces (body, base, weapon, shield, funny hat, squirrel on shoulder, ect.) Cost to print will be $8-15 for something at the 28mm scale. We add a small markup so we can stay in business, and then then there's cost of shipping, which changes depending on where you live. Based on this your first model would likely run you $35-ish dollars. Not bad considering people have paid hundreds for custom miniatures in the past!
The cool part is that once you buy a piece, you get to use it over and over for no extra cost, so your cost of components go down, reducing your next models price by a third if you re-use ones you're previously purchased. And if you have your own 3D printer you suddenly don't have to worry about our print costs or shipping either.
And those prices are just to start with, as technology gets better and we improve our service, we'll be able to bring costs down even more!