There is an alternative though, because I could get the Thunderbolt TM in Diamond/Pearl/Platinum, trade my Ampharos into it, teach it the move, and trade it back, since HGSS and DPPt are in the same generation. The way you get Thunderbolt in the Sinnoh games also involves paying 10k coins for it, but the slot machine minigame is still intact. The question is: would the slot machine get me to 10k coins faster than Voltorb Flip?
Absolutely 100% yes. So, the slot machine is obviously rigged because it's a slot machine. But it's rigged in a very bizarre way. You spend 3 coins per pull, and the max payout (besides the jackpot) is 15 coins. You choose when to stop each of the three reels using the Y, B, and A buttons. This all happens on the top screen of the DS; on the bottom, for some reason, is a picture of a moonlit field. Sometimes when you put your coins in, a random Poke Ball will appear in the field. This is very important, because it's completely impossible to win if there isn't a Poke Ball there, and if there is one there then you're guaranteed to win money on that pull. So the stuff with the reels doesn't even matter because the outcome is predetermined each pull anyway.
This makes it sound like you're not going to make any money at all playing this game, but au contraire! The other weird thing about this minigame is that a Clefairy could randomly pop out of one of the Poke Balls on the bottom screen. If this happens, then you're probably going to get the jackpot within the next few pulls, which by itself gives you 100 coins, and also starts the bonus round. In the bonus round, pulls only cost 1 coin, and Clefairy will point at each reel in the order that you need to stop them. If you follow her instructions, you'll win 15 coins each time. There's 15 pulls in a bonus round, so if you do everything correctly then you net 210 coins in the round. There's also a high chance that another bonus round will immediately start after that. So you just keep on following Clefairy's instructions until the bonus rounds finally end, and you win big money.
I've been trying this for a little while and it's very bizarre. I had a set of bonus rounds that netted me over 2000 coins, and the next set netted me over 3000. I lost something like 200 coins between these sets, before Clefairy showed up again. There's definitely randomness involved because you don't know when you're going to activate a bonus round, nor how many bonus rounds you'll get, but there's clearly a lot of money to be made here. You don't even have to look at the top screen, which is weird. Just press X repeatedly to spin the reels, press Y, B, A to stop them, and then spam X again to start the next round. Eventually you'll get into a bonus round, and you can just watch Clefairy. It's not entirely mindless because you do have to pay attention, but it is so much easier and more reliable and faster than Voltorb Flip.
It probably took me like two hours to do this but I got the Thunderbolt TM! The trade went well and I give all of my Pokemon very normal names:
