This isn’t WordPress-specific, but Andrew Youderian wrote an outstanding case study of his company’s $50,000 redesign. The redesign is for an eCommerce company and includes more than just the website design — it’s branding, migration from Magento to Shopify, and custom development for Shopify. You don’t see case studies this detailed that often.
Andrew was also a guest on Matt Report recently, if you want to learn what a relative WordPress outsider thinks about WordPress for eCommerce and why he chose what he chose. I think he’s wrong, but I’m a lot biased. I couldn’t find the link on Matt’s website, so here’s the Soundcloud; it’s a good interview.