Hi.
Personally I found Deep Space Homer to be the hardest figure to find. But in terms of highest price for a single figure, it was Glow in the dark Radioactive Homer.
You occasionally see prototypes on eBay. But you can't tell whether they're authentic or not. Perhaps if you're an expert or know someone who worked for Playmates and went ahead and bought some you could tell. I wouldn't recommend buying them though.
The Town Square environments (can't remember if the figures were aswell) were spread out between America, Australia and the UK. I've seen it go for $120.
To collect half of the figure within 2 weeks is pretty impressive. Good luck with completing your collection.