Stoecker (Industrial Refrigeration, ISBN 0-912524-42-1) suggests that two stage compression should seriously considered for evaporator temperatures below about -20°C (-4°F).
In practice, the decision to utilize 2-stage compression has alot to do with the load at both suction temperatures. If you have a large discrepancy between high and low side loads (e.g. really large freezer and small dock or really large cooler and small freezer), then a single stage economized system may provide enough energy efficiency to rule out 2-stage.
When the loads are significant and matching in scale, then a two-stage system may be more appropriate, given the additional first-cost of this type of system.
It will be difficult to prepare a spreadsheet that would truly take into account all of the variables, esp. if it is a true ROI evaluation. It would be useful to simulate a few different system types to see the trends, but every project should be evaluated on it's own merits before settling on a particular design. IIAR is a good start for contacting qualified refrigeration engineers to assist you (I am also a member).
-chillin