I've used Zencart to extremely good effect in the past. Free cart, skins to customise it are around 150US, but I'm yet to find a skin that is the perfect fit, they will usually require some adjustment.
Now Zen has a very good back end for admin, however if you are accepting money or storing customer's details on the site, then I would suggest getting some security checks in place as I am not sure about just how secure it, or any free site would be.
Having your own site developed will be the most expensive option, but you will get what exactly you want, and that's a massive bonus if you can afford it.