About 15 years ago, you could get one loose and almost complete for about $150 on eBay. MIB for ~$250. Of course, I was in high school then (like many of the people here, I'm sure) and didn't have that kind of money. Now that I do (again, like so many of the other MotU fans!), the price has gone up-up-up.
100% complete Eternia's are superhard to find and the parts are ridiculously expensive. My guess would be ~$500 for one loose & 90-100% complete and $1,000+ for one MIB. Although I'd love for it to be less.
Anyone else have better estimates?