Why would the XT prices have risen?
Until Soarer came along, and a $60 Hagstrom (the breakout board, not the nice self-contained unit they sell today) was the only game in town, XTs rarely sold for over $15-$20 + shipping.
Soarer's Converter (2012-ish?) made it a viable proposition, and prices went up to $25-$30. I sold some perhaps 3-4 years ago and it was difficult and took time to sell them on ebay in the $35 range.
My guess is that the current jump to $50-$80+, which occurred 1-2 years ago, is the result of continued repetition of the mantra "the XT is the best-feeling of all Model Fs" (which is arguably true), along with Orihalcon's cables.
In my opinion, considering the desirability and rarity of the AT, the low-mid-$200 range is about appropriate for those beauties.