There's too many possibilities, SEO could be thousands alone, Google marketing (AdWords?) depends on their budget, they could easily spend thousands dependent on product type, Facebook/MySpace/YouTube Marketing again that's so vague.
The 300 product element is strange too, I would suggest getting a price for an e-commerce CMS then if the client doesn't want to setup 300 products get that quoted/charged after or go to a service that will populate a site with products, I've seen those around.
How long is a piece of string!?