Hmmm, that's like saying what's the average cost to cook something in the microwave... The operating cost of the machine is a smaller fraction than the material cost - so your design and the material price will be a bigger factor in cost.
For example, our store, ToyBuilder Labs sells PLA and ABS at roughly $40/kg (we have some for as little as $25/kg - most are in the mid-$30's).
Depending on the object, you will need to adjust the material infill - hollow objects will be much less expensive than solid parts.