Shops are randomly-generated features that can be found in the caves in almost any level. They are small alcoves dug into the rock, typically manned by a Shopkeeper, selling various items or services. Shops can appear in almost any level, except for one-off areas or level 1-1.
If any tile that is considered to be a part of the Shop is damaged, or the player takes an item without paying, they may anger the shop owner, incurring the wrath of the vendor.
The cost of all items and services sold in shops increases as the player progresses through the levels. Prices are determined by a standard "base value", plus a "level inflation coefficient". The 'inflation coefficient' is the additional cost added to the base value, which is multiplied by the number of normal levels completed.
The formula for pricing is:
Base Value + (Levels Completed * Level Inflation Coefficient)
For example, the Bomb Bag has a base value of $2250, and the level inflation coefficient is $250.
Therefore, on level 1-2 (one completed level), the price is:
$2250 + (1 * $250) = $2500
On level 2-4, after seven completed levels, the price is:
$2250 + (7 * $250) = $4000
For almost all items, the level inflation coefficient tends to be 1/9 of the item's base value, with a base price divisible by 450.
For more specific information on the appearance and function of Shops in the Spelunky series, see the pages below.
For more information on the mechanics behind angering the shopkeepers, see the pages below. Note that in Spelunky 2, other shop owners, like Yang and Tun, are angered in the same ways as the shopkeepers are, although with varying repercussions in later levels.