I think a better way to put this is by the hour
$150-$250 per hour for a good designer
Check out
Welcome to Capital Auto Auction which has a lot of unique features including a complete auction function with user membership.
This site cost us $5000 upfront and maybe $2500 over the past two years for changes.
Now if you click the link on the far right of the main tool bar from the Capital Auto Auction Site named C.A.T.S. you will be taken to their inventory management site which they spent about $250,000 on over the past five years. I'm not saying it was money well spent... but you can get an idea as to how expensive some systems can be.
When you get into a little more complicated code and DB development the costs get a little higher. I'm just finishing a site that I spent a little over $12K on so they can get a little expensive. It all depends on what you need and how well you lay out your plan. A well developed flow chart that a programmer can use to develop your site can save you a ton of money.