Rules for shopping. They're designed as part of the Rogue Space project, but could be adapted.
What's Rogue Space? A free, compact, rules-light, modular RPG from Fenway5, which a few bloggers are expanding with ready-made systems and content. The key links are in the post footer and my original review is here.
Given Rogue Space is rules light, this is simple and fast too. The idea is to allow a GM to draw up a master equipment list for the campaign with just one price listed for each item, but have that price vary according to location, and suggest supply and demand.
Reference cost: Each item introduced to the campaign is given a reference cost by the GM. This should be the average minimum cost in a location where it is very common.
Rarity: The GM gives each item a Rating for its rarity in a given location or region, or the whole campaign. Rarity is measured on the WARE scale, for another RS-style acronym.
W = 1 = Widespread = A large quantity widely used and easily accessible.
A = 2 = Appropriate = A reasonable quantity used by or accessible to many.
R = 4 = Rare = A small quantity used by or accessible to specific groups.
E = 8 = Exceptional = A tiny quantity, or unique item; difficult to acquire and hold.
Example: A keyring reactor may be Appropriate on a technically advanced world, but Rare on the frontier. A fertile soil Widespread there may be Exceptional on an orbital.
Starting equipment: Core starting characters begin with any number of Widespread items, up to 1D3 Appropriate items and up to 1D2-1 Rare, i.e. W / 1D3 A / 1D2-1 R.
Finding supplies: More acronyms - the two basic approaches to finding supplies are What You See Is What You Get (WYSIWYG) and Garbage In, Garbage Out (GIGO).
Using WYSIWYG, 2D6 is rolled and the Aquiring Attribute added. The GM may wish to apply a modifier based on the nature of the search or centre. The result is compared to the WARE scale. For each Rating equalled or exceeded, one item type of that Rating is available. For each multiple by which a Rating is exceeded, one extra type is found. The number is also the number of each type. Player and GM alternate selecting these types.
Example: A result of 6 exceeds the Widespread Rating (1) by 5, the Appropriate (2) by 4 and the Rare (4) by 2 and so turns up six items of six different Widespread types, three items of three Appropriate types and one item of a Rare, i.e. 46 items of 10 types.
Using GIGO, one item of any type and Rarity may be found by means of a successful Aquiring test. The type sought must be specified in advance of the roll and the target number is the rarity Rating. The GM may wish to apply a modifier based on the nature of the search or centre. For each multiple by which the target number is exceeded, one extra item is found. The GM may allow consolation items to be found using WYSIWYG.
Example: If the target number is 4, a result of 9 exceeds it by 5, turning up two items.
Pricing: To find the price, roll a number of D6s equal to the number of items of that type currently available and discard all but the lowest result; multiply by the reference cost.
Example: A reactor keyring has a reference cost of 50 CR. If three are available, three dice are rolled, for a 5, 4 and 2. The 2 is used: each item costs 2 x 50 CR, i.e. 100 CR.
Update: This has been reorganised, revised and expanded for Rogue Transmission #1.