A bit of reasearch in the past few minutes shows the tangelo to be a hardy tree in warm climates.
I'm still now sure why the price is sky high.
Here is what one man thought about the price :
http://www.sicklesmarket.com/Newsletter/Archives/FN_194_February_1_2003.htm " While at the market I noticed that Mineola Tangelos were available from both Florida and Arizona. Those from Florida were decidedly more expensive. Being curious I bought some of the Florida ones and compared them to the Arizona tangelos that we had back at the market. There was a marked difference, thus justifying the high price difference. The flesh was more tender, the fragrance more intense, and the juice just ran down my chin! In fact, Mineolas make the best orange juice you will ever have (despite the fact that it becomes a relatively expensive juice). "