The following statistics are based on my understanding of how the not-open-source Zelda 2 Randomizer works, and do not necessarily reflect reality in any way.

Spell Costs

I think spell costs are +/- 50% of the base cost OR the previous level's cost, whichever is lower. Note that spells cap at 128, but I don't factor that into my calculations because it's kind of irrelevant unless you find all magic containers before the cost drops. Consequently, the numbers below are technically medians, not means.

These costs are calculated when the page is loaded based on ten thousand trials, and may change when the page is refreshed.

Spell12345678
Shield3122191412111010
Jump483528241814107
Life7058494439363432
Fairy8066524636312927
Fire11976552915131110
Reflect11999734635282114
Spell12096806746302115
Thunder11999898379767158
Spell12345678
Shield-1-2-5-2-4-5-6-6
Jump0-5-4-8-2-2-2-1
Life0-12-11-16-11-14-16-18
Fairy0-14-8-14-4-9-11-13
Fire-1-4-5-1-1-3-5-6
Reflect-1-21-7-2-5-4-3-2
Spell0-16-16-13-2-2-3-1
Thunder-1-21-31-37-41-44-29-6

Attack Effectivity

I think attack levels are +/- 50% of the base damage OR the previous level's cost, whichever is higher. The only levels whose averages are really affected by the randomizer logic are the weaker levels (3, 6, 8), as these levels provide a 25% boost of power, and the other five levels provide a 33% boost.
LevelDamageIncrease% Increase
12
22.80.840%
34.21.450%
46.2247%
592.845%
612.63.640%
718.6647%
825.46.836%

EXP Requirements

"This shuffles how much experience is needed for each level. The values range from -25% to +25% of their normal values. It is guaranteed that each level will cost more than the previous level."

I don't know what "more than the previous level" means, so I'm using the same logic as the above: If the previous level costs more, raise the cost of this one to match it. Maybe the randomizer re-rolls? Anyway, since the 75%-125% range is so small, it looks like the only average affected by the randomizer is Attack 9. Shout-outs to Atk9Guy.

Experience12345678
Attack200490100020003000500080209330
Magic9029070012002190350060008990
Life401404007901500249040008990
Experience12345678
Attack0-10000020330
Magic-10-1000-1000-10
Life-10-100-100-100-10