An ecommerce site? Not a huge site, but a few categories, each with several products. Needs picture, description, shopping cart, and option to choose a size S-XL.
You could probably make a landing page in HTML pretty cheap and then use a free shopping cart like OSCommerce to do the rest. Tie them together with some links and you'll be all set.
If you are ok with learning your way around installing scripts to build your ecommerce site with, then can handle basic HTML to customize pages that need fine touching, then it might cost you nothing. There are lots of open source ecommerce software available if you google.
I agree with clive. Having searched ror many scripts myself at times, I think you could find a script to handle an ecommerce site and keep it organized and nice looking for cheap if not free.
It should cost mid - $$$, upwards. You will pay more for a decent design, and for custom code prices will probably start in the thousands. You really have to ask yourself how much you need a unique look for your store: this will inspire confidence and may help you to sell more. It just depends what you want to sell and on what kind of scale.
Can start all the way from $o.oo to ...any amount. Just depends on a lot of things. You can get a free Oscommerce template and if you know enough coding you can build from there on your own. There are other ecommerce templates low budget that you can work from. If you need to pay, can start from $100 for a reasonably good job from a quality webmaster.