It depends on your needs really… It is hard to just give you an exact answer.
The cost of actually installing Magento - FREE. As its Open Source software (Community edition)
The cost of templates, you can find them on: http://themeforest.net/category/ecommerce/magento
Then there is setting up everything, menus, categories,… And using extensions for Magento, you may customize your site to your liking… Some of them are free, some are paid for…
There is also the cost of shared/VPS/Dedicated hosting… Which varies according to your needs.
We will be more than happy to help you out if you need anything… Just send us a private message