Sorry for the delay, I was out on a break. You don't want to put any additives into 40/50 series transmissions except as a last resort.
The SPIII from Mitsubishi is about $8 per quart list price. The Hyundai SPIII is often closer to $6-6.50 last I checked (I only work for Mitsubishi, so I'm not on top of Hyundais pricing).
There is no "good" replacement from the auto parts store. Just about all parts stores will direct you to either Dexron 3 or a universal ("multi car") fluid. Neither are acceptable long term fluids, especially if you are potentially chasing a fluid related issue.
There is one specific SPIII replacement made by NAPA. I have heard mixed reviews about it, and have to take into consideration that they are basing it on the SPII fluid (The same way the parts stores recommend Dexron and Dexron-base universal fluids).
The reason these are at issue is because of the fluid hierarchy. In a nutshell:
Originally Mitsubishi had SPI. It superseded to SPII.
SPII was compatible with Dexron 2.
Dexron 2 superseded to Dexron 3 (and became the base for universal fluids).
Dexron 2 and Dexron 3 are forward and backward compatible.
SPII superseded to SPIII.
SPIII is backward compatible with SPII, but SPII is not forward compatible with SPIII.
And so... Dexron 3 meets SPII requirements, but not SPIII.
Specifically, there is a different friction coefficient in the SPIII which causes the Dexron base fluids to slip a small amount more than designed. This results in increased friction wear and increased sediment as time passes. Not too much of an issue in the short term (unless you already have a slip issue then it can aggravate it), but in the long term it makes a normally unlimited mile transmission with regular maintenance a 50k-75k mile before frictions wear out and you need an overhaul type transmission.