I guess it'd be a two-foot high vinyl kit of Godzilla (Bio-Goji) from Paradise, for which I paid $275 USD. That was before I moved to Japan...and found that the same kit retails here for about $100!! Oh well, what did I know...(I knew that it was
super-cool!).
No worries, though; I had sold an 18" Kenner "Alien" figure for $250 to finance the Big G, so I actually invested only $25 in it!
As far as the highest price paid for an in-production kit in its normal market, it would have to be either my 1/16 Tamiya Tiger I or Tiger II static kits, both of which cost 23,000 yen (about $210 USD). Actually, the Aber detail-up set for the Tiger I cost more than the kit! It was 29,800 yen (about $270 USD). So, the Tiger I
PLUS the Aber set was about...$480? Still cheaper than the RC version!