As has been said there are a number of different factors which need to be considered but the straight answer is probably not too much. To be honest I think a lot of it depends on your client base. If you are already an established company which is just looking to sell online then much of the expensive marketing costs will already be taken care of whereas if you are actually starting up from scratch then you will have to put in a lot of groundwork before you see any solid results. Hosting and e-commerce software is getting cheaper and cheaper as more competitors enter the market though so nowadays the actual capital start up cost is relatively low.
If you have any more questions then send me a message :)

