A typical SEO eCommerce website costs about $1000, plus about $2-$5.00 per product (depending on complexity), and 2.9% for accepting credit cards online. I've been charging $1000 for eCommerce websites upgrades, plus $50/hour for further upgrades and updates. Look at the examples of eCommerce websites listed below.
The $1000 includes everything you need to promote your business and sell products online: a custom layout created in Photoshop matching your logo, custom HTML website template, about a dozen basic web pages with multimedia, customized PayPal buttons and shopping cart, company pages on every major social network (including Google+ Plus, Facebook, Twitter, Pinterest, YouTube), and a WordPress news blog to supplement the website (beware of amateurs trying to sell WordPress blogs as commercial "websites").
We are authorized by PayPal to create custom "Buy Now" buttons (custom graphics and custom HTML code), which have exceptional marketing potential with my SEO websites. There is no set-up fee; no deposit; no monthly fees; and no other merchant accounts are necessary. Vendors only pay 2.9% to accept credit cards (payments with PayPal are free). The shopping cart is free - but it can be upgraded. The mobile card swiper is also free and works as a stand-alone Point-Of-Sale. You can even get a PayPal MasterCard to purchase materials directly from your account.
We can also create eCommerce websites on any platform. One of the most basic eCommerce platforms is Shopify on the WordPress platform - the drawback vs a SEO website is that when you use a WordPress blog (which are free for anyone) your website will be 90% identical to every other blog on the internet (the same goes for any cookie-cutter platform). We prefer to use an SEO website and a WordPress blog together to generate maximum sales.