I am going on a business trip to Amsterdam at the end of March and am planning a sidetrip to an other city within a "train distance" from there (since I have been to Amsterdam various times). In general, what cities in Europe are the cheapest to fly to from Miami in March?
If you are looking for open-jaw flights, you are probably going to find that your options are limited to airlines that fly from Miami to Amsterdam - then see what other European destinations they fly to. There is no "general" about airline prices. They fluctuate for no apparent reason.
According to farecompare.com, the cheapest European destinations friom Miami in March are Amsterdam, Stockholm, and Rome. Milan & Venice qualify too.