MEM90 addresses are usually used for pointer searches. Also, in Wii memory, the addresses almost always end within 4 byte segments (0, 4, 8, C), not 3, but that is probably because this is MEM90, not MEM80. You said value, did you mean to say address instead? Values are what are held at an address. An address is just a location in memory to hold a value / instruction.
If you're using Dolphin for RAM dumping, assuming you have edited the target properties of the Dolphin shortcut to enable Dolphin into debugging mode, click on MRAM for MEM80 and EXRAM for MEM90. This is easily memorized by MainRAM and EXtraRAM.
Changing the first value (address?) from a decrement (what it is doing) to an increment with assembly would eventually give you more than the max. I don't know what "potara" is in the game, but let's say it's cash, just for this example - Incrementing instead of decrementing, upon purchasing something, will increase your money instead of decreasing.
The second address, as you said, is the cost. So, this, depnding on the instruction(s), can be done either with a 32 bit write or an assembly instruction. If the game holds the cost at that address for each item, assembly will be needed to tell it what the cost should be for all items. Moreover, if the address holds the value for a single item, (the potara), a simple 32 bit write should suffice.
If this was 6502 assembly for the NES, I'd be able to help you out a lot more, lol Unfortunately, I know very little to none at all PowerPC assembly. The above paragraph is solely based off of my experience of game hacking. It might not even be correct. You'll have to do some experimenting. Every game is programmed a bit differently, but the concepts to game hacking still apply in all regards, nevertheless.
Also, there is no problem with double, or even triple posting, so long as you are contributing information to the topic, and not just a few words (spam).
I hope this gives you a bit more guidance. If you have any more questions, feel free to ask. Always here to help.