No, building a well designed shipping container structure is not necessarily cheaper than traditional buildings. You still have to design to code, pull permits, choose your interior finishes, fabricate, and ship the shipping containers to their final destination. They are cheaper in the long-term, because for the most part, you can move them to your next land location - try doing that with traditional structures. The ability to reuse a modular building, greatly reduces the cost for redevelopment.
If you are considering a retail space, pop up shops are a popular way to test your concepts. Much like a roving restaurant - also known as a food truck - containers are a great way to show proof of concept, before committing to a permanent buildout and pouring thousands of dollars into a new business or expansion.