A dead engine sounds typical of an electrical problem. Spark plugs, distributor, alternator, or ignition coil can all contribute to your problem. You may even have a clogged fuel filter.

Indeed a dead ECM is possible. ECMs can die from excess moisture, power surges, or even poor design from Mitsubishi. Certain Mitsu models had ECMs that had incorrect capacitors in them. You can always pull the ECM out and have another garage test it for you. A second option could save you lots of money and headaches if your ECM is not the problem.

Your ECM can be expensive because the Mighty Max isn't a very common Mitsu model on the road. If you do in fact need another ECM, call around to any auto junkyards in your area to see if they have Mighy Max trucks with your same engine. If you can find a working ECM from a junkyard, they will probably only charge you $20 if you take it out yourself. You can even search the Internet for third-party rebuilders of ECMs. Many times, you can get one at a very reasonable price with the exchange of your dead ECM.

If you don't want to bother looking around, call your local Mitsubishi dealership for the cost of a ECM. Be sure to write down your VIN when you call them. They can usually find parts quicker for your car when you give them a VIN.
Good luck!