The design, css and html is the cheap bit. It's the backend which costs the money. That is, the ability to design and setup a database to store the data which is the images and which gallery they are in, the text associated with each image, the price and shipping cost of each image. Also the accounts records for each sale which also means an account for each customer with a log in so they can check whats been happening etc etc etc.
Then you have to build the content management system to handle all of that which is more design, css, html and probably PHP or ASP coding to manage all of that which means advanced online forms processing with insert, edit and delete options where necessary. Then you need more php or asp code to build the actual web page content dynamically with buttons for buying which also needs coding to interface to a payement gateway including collecting sucessful payment data and subsequent update of your own database with that client payment data etc etc etc.
To put it bluntly, anyone who thinks that a simple website for selling images is a breeze to knock up, doesn't know what they are talking about. It's simple relatively speaking with regard to some really complex e-commerce sites but its still a lot of work and is beyond your average hacker who just tries to bolt together some free utilities they got there hands on.
That's why a bespoke package is usually a better option because they have already done the hard work and refined it over time so you get a decent content management system and they provide some templates you can tweak for your own look. They make their money on selling the work many times over so you get a good a price. Go for a bespoke from the ground up package and you pay big bucks for an unrefined system.