Every item has a "base price". You'll probably be familiar with this from second-hand stores or on-line auction sites. A common "base price" is £0.99 and as an example a second-hand PS2 game will usually have a "base price" of £0.99. However, the "base price" fluxuates between machines and formats. Again, you'll probably be instinctively familiar with this from places such as eBay. A PS2 game may be listed initially at £0.99; a N64 cartridge will be listed initially for more.
Choose a machine type (and format) below and you will be told the "base price"...
However, as we all know, some games are worth a lot less/more than others.