There is no one answer to "How much would it cost", as you would see below:
1. What you are trying to build is fairly straight forward, at the face of it - A shopping cart, order management system, a customization applet. It could get complex from a business perspective because you suddenly have a long tail of SKUs for 'made to order' requests. That's not a technology problem.
If what is described about is what is required, you are taking about a total of not more than 320 - 350 hours of work. Depending on where you get this done and what quality you would end up spending between 7000$ to 15000$
2. If your ambition is to build a community on top (designing a shoe is a form of artistic expression and your users might want to flaunt it on their FB page). You might also want to autopost cool designs to Pinterest (and thereby drive traffic back to your site). Now we are talking about 400+ hours
3. Add on top, a way for users to sell their designs and earn some royalty - You are now adding another 100 hours.
In other words, think of how all you'd scale this idea and what it means in terms of the expectations you'd place on your developer and then choose a developer wisely, so that they can be with you through the journey.
Ping me on PM or email me (email@example.com) and I shall help you find a good developer outfit. Gratis!