if you just want a web presence then there is plenty of free and very quick and simple to set up software available which provides your content management system.
for example, just about any blogging software allows this. Wordpress is one and has a plethora of photoblog/gallery plugins and you don't need any html/css/php knowledge to do it. However, you get what you get. If you do have a little php and html/css knowledge then the skies the limit as regards what you can do with it.
Putting paypal buy now buttons in a website is very easy.
But if you want totally bespoke design then it will cost. How much depends on your designer and what you ask for.
basically paying for a new content management system to be designed and built is like asking for the wheel to be reinvented. There are so many out there that that part of your website should not be costing you a lot of money.
On the other hand if you want a full on image asset management tool linked to your website then it will cost quite a lot for an off the shelf product and a huge amount to have one built for you.
If you are serious about selling online then look up "image asset management" in google and find the products which include web site generators in them.
just to get you started:
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the following 2 are high end image asset management companies
http://www.citdigital.com/
http://www.orangelogic.com/
last one is more a online gallery with sales options:
http://www.lightboxphoto.com/
all of the above would be a lot cheaper than having the equivalent built from scratch.
this last one is a managed option. They provide the hosting and webspace management. you just pay a yearly fee and pick and choose from a host of templates.
http://www.photium.com/
Unless you are really serious and have real money to spend then I would opt for one of the free options unless you are confident and have the time to spend on learning and developing your own site.