The math behind those bonuses are more complex than it looks. The way it works is very different comparing to slots from 20 years ago, where the bonus is as simple as "12 freespins, all winning pays 2x". I'd assume some tweaks are being made to make a more balanced RTP/experience.
Here's some examples.
ELK studios:
Almost all of their regular bonus have a price of 100x, and all super bonus have a price of 500x. But the additional privilige from those super bonus varies. From a much better beginning such as Max rows, Both Ways on Nitropolis 3/4, to guaranteed super wild strike on Propaganda and Katmandu X, or a sticky multiplier in Ashoka. But after all those effects, the price of super bonus remains 5 times the price of regular bonus, and both comes with the same RTP.
Nitropolis 2 is another great example. If you buy a super bonus, you get 2 nitro reels on all 6 reels. Each of them have 14 possible results: 12 regular symbols, wild, and scatter. So you need a symbol appears on both reel 1, 2 and 3 to win, each reel contains up to 2 types of symbol among 14 possibilities, if there are no tweaks being made, it's likely to cost an unreasonable amount of time to get a win. And we all know winning by getting multiple "Nitro Match" or "Nitro Wild" in a row is pretty hard.
BTG: Many of their Megaways game comes with a sticky multiplier during the bonus, and it increases as the bonus goes. This should make each spin more valuable than the previous one, and the difference between a bonus begins with 20 spins should be much more valuable than two bonus that begins with 10 spins.
Now let's take a look at Vegas Megaways. Here you can buy 10 freespins for 75x, or 20 freespins for 150x. This makes me really concerned, if there are sticky multipliers that increases after each win, how could 20 freespins worth only twice as much as 10 freespins? For this to happen, some tweaks must be done in the background, so that the HF (Hit Frequency) become somewhat lower when one buy the 20 freespins.
