also factor in the mono vs duo for the horned frog portion, which is not to be tossed out. monos are notably easier to pull off than duos. and the percents are a bit off because each proc is independent, so it isnt simply additive, there will be a 27/64 chance that at least one thunderstorm will be cast when using adrenaline, and a 37/64 chance that none of the attacks will proc a thunderstorm. It's a little less than a 50% chance when fleshed out, but there are also chances for 2 or 3 strikes that turn as well, albeit much lower percents. (link for probability related math:
http://www.edcollins.com/backgammon/diceprob.htm )
as far as the mummy goes, that's a more straightforward comparison, but you may have made a typo with
"So +1 cost, - Mummy, 3->2hp = 0.75 Thunderstorms per turn?" -> "So +1 cost, - Mummy, 3->2hp = 0.25 Thunderstorms per turn?"?
and yeah, i think 1 cost for a 25% chance to proc a single 1 aoe damage is worth about right. keeping in mind something such as a plague would proc 1 damage aoe to all creatures each turn for 2 (be it from the retrovirus or the spell itself)