Basic Spree framework is available for free – it’s open source.
But you need to customize it to your specific needs and that can be done by a specialized Spree shop. Or you can work on it yourself – if you can spare the time. Just like with any other open source solution it saves you so much time and let’s you focus on your business-specific development.
If you hire someone the initial cost will depend on:
- who you hire – of course – you get what you pay for – that’s another topic for discussion – see here: Michal Faber’s answer to Which service companies are good at building outsourced RoR development teams for startups?
- extent of back-end customization work – how much you want to differentiate yourself from the competition, how much industry or business specific requirements you have (fashion / apparel is different from electro parts)
- extent of front-end / layout work – how pretty and custom your website should look & feel, how much fireworks improving user experience you want
- extent of integration work – payment gateways, shipping integrations, others like mailing solutions, invoicing, inventory & stock information management and so on
- scalability & hosting requirements – how much traffic you have or expect and how fast is it going to increase
And that’s the costs of launching an MVP / your website initial version.
Then you launch it, market it, observe what get’s traction and what doesn’t, your requirements build up, your customers submit theirs – development never stops if you want your business to grow. It all costs you money but at least you have an MVP up & running to test your market and invest wisely to maximize your returns.
Hope it helps. In case you have questions go ahead and drop me a line.
And here’s a great discussion between CEOs of Shopify and Spree Commerce which made my day 
Right to the point however biased:
Should an e-commerce startup start off with an open source solution (e.g., SpreeCommerce), or build a portal from scratch using, e.g., Rails? Why?
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