while I can't comment on the pricing but it is important to note that you should ink your support scope accurately and as clearly as possible. I have been maintaining my own ecommerce sites and as an admin myself, I've always wanted to add in extra functionalities. Often I find myself almost overrunning my budget purchasing addons ... etc.

Also as mentioned by trickyhost "eCommerce can be implemented in a day, however for people to find your site, it may take months." SEO is very important and this is very costly to maintain. I guess to a certain extent developers still needs to provide a certain level of SEO, eg. on page optimization, link submission ... etc. This will incur additional costs if clients want ongoing campaigns.