Tied to locations are vendor lists which have relative chance values for each item listed. Locations also have a max item number and a money value. So, if wheatstraw and AI Core both had the same relative chance to spawn, the actual likelihood of an AI Core would be much lower because of the money limit.

Does anyone know of a good way to calculate spawn rates for items? 

Is it important to wiki users that there are percentages listed for items rather than simply using the categorical labels of "guarenteed", "common", and "rare"?