Yeah, that's kind of a shame. You might not need quite a multi-million dollar budget, but you probably do need a couple hundred-thousand, minimum. Instead of buying a lot of this stuff, you could rent. That'd save you money, but it's still a nice chunk of change.
It'd be nice if more people had a chance to find it. But, it probably parallels what's going on with the main story, with something important being hidden at the bottom of the sea. It makes sense to do things this way, but I wish the playing field was a bit more level. Of course...I'm not entirely sure if this was actually supposed to be a "challenge" in the first place. But, Oda knows what he did.
Mind you, there is a chance that someone really creative could find a way to get it, without spending that much money. There is the famous example of raising a sunken ship with ping-pong balls. Sometimes creativity (or insanity) can work instead of money.
Maybe someone could use a pack of trained dolphins to pull it to shore for them, or something? Sometimes you need to think outside the box.
The biggest problems, as I see them, are:
1) Actually getting to Japan in the first place. If you're not already there.
2) Actually confirming the location where the treasure is.
3) Actually raising the "treasure" back up to the surface.
4) Actually unlocking the device without destroying what's inside.
5) Actually being able to read it, if you can't read Japanese.
6) Actually getting away with all this, without getting the Japanese coast guard or other government agency mad at you, and getting thrown in jail.
Problems 2 and 3 are probably the most expensive parts of this process. As Problem 2 requires a lot of educated guesswork and scouting, causing a lot of travel expenses. And Problem 3 requires specialized skills and equipment that most people don't have.
The online fans seem to have a pretty good handle on the general area where the treasure is. But, this isn't a huge thing you're trying to find. So, finding something relatively small at the bottom of the ocean is still a pretty tall order. Still, if the online fans are right, you may not actually need to spend that much on Problem 2.
So, that leaves Problem 3 as the big money sink. And I'm not sure how you get around that. Even renting a sub or a specialty salvage boat of some kind is pretty expensive. If you could find some way to hook it, or maybe magnetize it, maybe you could figure out something cheaper? But, even looking that far down there with just some sort of remote camera is kind of a tall order.
So, yeah. It's probably going to be someone rich who ends up finding it. Unless Oda gets wind of it, and decides he wants some Japanese kid to find it, instead. So he just let's the Japanese coast guard "help" a kid to be the one to find the treasure. I could see Oda doing that for someone from his hometown or something. Keeping it in Japan, instead of letting a rich foreigner be the one to get it.