Here is my personal experience with bonus buys during the week. I've spent around 15k EUR. These were from winnings on Crash and roulette, and they all gone. I've bought over 500 bonuses across games including Fire in the Hole 2,
Deadwood,
Money Train 2, 3, Sugar Rush, Big Bass, Sweet Bonanza, a few other useless Pragmatic slots, as well as some others that i can't even recall the names of.
From what I've seen, in the typical case, you don't win much more than the initial bonus purchase price.
The worst for buying bonuses from the mentioned slots was those from Pragmatic Play,
like in Sugar Rush you can easily get under 10x from even 20 spins. Out of +/-30 bonuses, you get some odd 300x win.
Fire in the Hole 2 was pretty safe to buy, but only when buying the bonus that costs 600x (20p bonus costs 120 EUR). In this bonus, it generally pays between 200x to 1000x. This is my observation from buying around 200 bonuses ranging from 20p a bet to 1 EUR. Those cheaper ones were pretty pointless. I also tried twice the most expensive one for 3600x a bet, and both paid under 800x...
The first spin is a decisive one in any of its bonuses. If you land diamonds on the right and that nutter jumps off the trolley and becomes a persistent collector along with a dynamite that smashes the box underneath him - you can start opening a champagne cork. But this likely happens rarer than pigs usually take off from Gatwick airport.
In the top bonus (3600x), that nutter gets thrown out right away, but don't get too excited as you might not get any dynamites, and your money simply will be flushed down the toilet.
The base game in Fire Hole was quite OK. I was playing it for 4 hours at 20p with a 100 EUR deposit. I guess it could have lasted even longer, but i started to do bonus buys and lost.
Natural bonuses were also landing pretty often, but in that basic bonus stage, I think one must be extremely lucky to get something decent. For a good start, you need a persistent dynamite to smash all those stones and boxes below.
Surprisingly, however, the basic bonus - my first-ever bonus purchase on this slot, paid the most, and it gave me a balance to do more buys on other slots.
The Money Train's 3 basic bonus was also pretty safe (i mean you don't generally get 10x wins like in Pragmatic's), but you rarely win something good enough on top; often it's just 110x or a liiittle bit more.
To win something decent, you need a 500x bonus buy and get a good persistent symbol, like the Sniper which gets chosen rarely (see below). Otherwise, expect wins from a 500x buy to range from 150x to 300x.
Pragmatic's Sugar Rush was a piss-take as usual -
When you're doing many buys - like 10-20 in a row, and constantly adjust stakes based on previous wins/losses - it starts to seem like the slot knows you have enough cash and are trying to beat it, and it doesn't let you get ahead. Often it simply goes by a pattern.
Say now you get a 300x win, then 3 wins under 100x follow, then you get one that is a bit over 100x, then again one under 100x, and finally one that is around 200x. Overall, it's like stumping in your own shit - you don't win anything but gradually lose, lol... So essentially, these are pointless feature buys.
So to buy or not to buy bonuses? It's up to you, but treat them as buying expensive lottery tickets that are generally pointless to buy,

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If you have an affordable balance, i reckon, it's way better to increase a bet size for a bit to see how it goes than spend money on feature buys.