You can't negotiate by filling out a form, after all, and a sales person may know about conditions, new products or price changes that aren't posted on the Web yet. Many people feel their credit card information is more secure that way, too, and given how easily most e-commerce sites are hacked, they're probably right. Cost varies in the extreme depending on who is doing the work and how that work is being accounted for. If it can be done as part of normal operations, so much the better. If you can apply volunteer labor, so much the better.
Costs vary in the extreme depending on scale and approach.
Here the Web site is linked to or integrated with the company's back office processes. The on-line catalog is built dynamically from a database. A couple million dollars will get your started. A few billion bucks will get you into this game, but doesn't guarantee success. Steps to Success Business Strategy Planning is the first step, and absolutely essential unless you are fond of flushing money down sewers.