Yes, you can make them available.
First, their cost is identical to the value found in <ShopValue> tag. A portion of that is also used when selling the item, so keep that in mind (you want to avoid a situation where a player finds an item then sells it for huge cash in start of game).
Second, how is an item unlocked? There are two ways to unlock items in the shops. The first is that they are always there, you can see examples in CoreImprovements.xml with xml like this
<UnlocksShopItem>SaltedPork</UnlocksShopItem>
the second way is to have them unlocked from a technology. For example in Burning Axe we see this
<Prereq>
<Type>Tech</Type>
<Attribute>Enchantment</Attribute>
</Prereq>
meaning the item is unlocked by that technology (note it is the internalname of the techdef, not always identical to what you see in-game; see Techs_Amarian.xml).
The simplest way would be to take an existing gameitemtype and just insert a prereq for a technology like above. You can further limit the item if, for example, you want it to be only available to certain races or factions with certain faction abilities.
