E-commerce and free don't really go hand in hand. Premium themes will have way more functionality and customisation than a free alternative and when you're dealing with payment gateways there will be some cost involved somewhere. That being said you could buy all the bits you need for a woocommerce site for around £150 (that's buying the bits, not paying for a developers expertise putting them together).
Something to consider with an e-com site is how bigger store you're looking to build? I've found that woocommerce is great for up to 20 basic products but over that, you should be looking at magento.