People tend to ignore the inherent costs in doing custom design. First up, you got the cost of labour to design and prototype the device. This can be variable as there are risks involved. Then you have the costs of compliance (CE etc) - this is not trivial. After that you have the cost of manufacture, testing and warranty. If you find a problem in the field, then you have the engineering and rework costs.

That said, stepper motors aren't exactly quite what you'de hope for in a motor as many people have found out the hard way. At first glance they're very simple - and that is where it ends. With any motor, you need to understand the load you are going to place on it - do you need high start up torque? wide torque over a large speed range? Steppers don't do this. Torque drops off quickly with increasing speed and start torque is low. Then there is the problem of resonances at certain speeds.

I would suggest you prototype your machine with 'off the shelf' equipment then get someone to come up with a custom solution - otherwise you'll be trying to solve multiple problems at once.