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.