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
Jump473528241814107
Life6958494439363432
Fairy7966524636312927
Fire12076552915131110
Reflect120100734635282115
Spell12096796746302115
Thunder119100898379767258
Spell12345678
Shield-1-2-5-2-4-5-6-6
Jump-1-5-4-8-2-2-2-1
Life-1-12-11-16-11-14-16-18
Fairy-1-14-8-14-4-9-11-13
Fire0-4-5-1-1-3-5-6
Reflect0-20-7-2-5-4-3-1
Spell0-16-17-13-2-2-3-1
Thunder-1-20-31-37-41-44-28-6

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%
46.32.150%
592.742%
612.73.741%
718.86.148%
825.46.635%

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
Attack200500100020003010500079909310
Magic9030069012002190350059709000
Life401404008001490249039809010
Experience12345678
Attack0000100-10310
Magic-100-100-100-300
Life-10-1000-10-10-2010