I should have thought of this much earlier, but I started to record which random Event I was getting in an attempt to figure out how often they show up. This is the data from the last 400 times I chose it. I ran a couple dummy runs to get to a round number but I've now collected all the achievements and probably won't update it further.

I've included the actual percentages (PCT) , and also my estimates as to what the programmed percentages are (EST PCT). My working theory is that there are essentially four ranks of event rarity: common, uncommon, rare and super rare. This is just my conjecture, colored by the thousands of times I encountered the events previous to the data collection.

Some notes on my shorthand:

→ : indicates that the resources preceding the arrow will be lost, and the resources following the arrow will be gained

X GOLD : indicates that the Gold amount will vary based on the Level and Rarity of the monster or trap being considered

FREE* : indicates that the cost is free if you happen to have a randomly selected monster in your roster