I've made several opencart shops and it is relatively easy to set up. You'd probably end up paying for a theme, and the best ones cost around $50 - $60 so they are not expensive either. Adding mods can extend its features and they vary from free to $30+.So a good theme, and a couple of mods and still it's only about $100.
The better themes do require a bit of work to get them up to speed as they offer so many options, but then again, that's what makes them the better themes. Many of the simpler and free themes are perfectly acceptable to be honest. One client had a server upgrade the version of php which killed his shop (an early version, 1.2 I think) and I found a free theme almost identical to his paid one.