Notice the question differs from “what is the cost of starting up a web-store?”. The first scenario forces us to look at the web-store as a real brick and mortar store (which it is), but the second question gives absurd results, after all, if you ask a freelancer he might say ‘under 10 grand’ but if you ask IBM they’ll definitely not say ‘under 10 grand’.
So what is a retailer to do? Turns out the way to calculate needed investment is quite simple. Look at the non SEM traffic your web-store gets and compare it to any one of your physical locations. If the non SEM traffic to your store is double of what your average store gets in daily walk-ins then you ought to spend double the cost of opening that store.
Only after retailers get serious with their eCommerce channels will they start to see revenue disruption.