You can set a default price for a single photo in the specified gallery and/or set a total price for the whole gallery. Therefore your clients will have option whether to buy several particular photos from that gallery or to buy the entire gallery.
You can also manually "override" the default price for a particular photo in the gallery, when you feel that photo has a greater value among others.
IMPORTANT: The price is always linked to the license. Therefore, before putting any content on sale, please create all the required types of licenses via the "Settings" -> "Licenses" menu item.