No idea why it costs so much. Teleport does everything Fly can do and more, and for 1/2 the price.

As for absolute usefulness, I would say it's tied with Move+3. In some situations it is far more useful, some it is far less.

(For those of you who don't know how Teleport works, you have a 100% success rate for any move within your normal movement range, and for every panel further you try and move the success rate drops 10%. So if you have a move of 5 you can Fly anywhere in 5 panels, and Teleport 100% anywhere in 5 panels. The advantage is you can try to Teleport 7 panels away with 80% success. Teleport also illuminates every square in the Deep Dungeon.)