When two treaties provide the same outcome, one can combine their different EP and $ costs with some basic probability calculations to create decision rules. Here are two examples.
For creating Trade Routes between factions: Trade Summit, 200EP, $200, 75% success Trade Meeting, 100ep, $100, 50% success (Two successive Trade Meetings have a 75% success) In this case, Trade Meeting is preferable. If the first one succeeds you save money and ep--and saving ep means you save time. If the first one fails, the cost and odds of success for running two is the same as Trade Summit.
For improving Trade Routes into Trade Alliances: Trade Alliance, 250ep, $1,000, 65% success (Two successive Trade Alliances cost 500ep, $2,000 and have 87.75% success.) Trade Treaty, 200ep, $400, 50% success (Two successive Trade Treaties have a 75% chance of success, three successive cost 600ep, $1,200 and have a 87.5% success) Results are a bit mixed here. The relatively small increase in ep required by Trade Alliances make them marginally preferable if you are in a hurry for success. In contrast, if you are trying to conserve credits, Trade Treaty is preferable.
Last Edit: Dec 28, 2015 13:07:41 GMT -5 by tenbsmith
That's a good point resistor, my analysis is more applicable for early to mid-game. Once an empire is larger, making lots of $ and pumping out enough EP to complete either treaty in a single turn, the more expensive one will generally be better.
My analyses in the OP also fails to account for the extra income associated with a treaty succeeding earlier.
Still, my analysis in the original post gives you a sense of the trade-offs in early to mid game.