1) Cost Per Acquisition (use this to figure out the average cost of a conversion):
Cost/Number of Conversions
2) Simple ROI (use this to calculate the profit on an investment):
3) Median: Order your list of values (for example donation amounts) and find the middle value. This tells you that 50% of the distribution is below that number and 50% is above it.
For lists that are highly skewed (where a few very high/low values radically change the average), this will give you a better idea of the "true" expected value of an action - such as a donation.