To me as a relatively inexperienced player, it seems a bit cheap to get permanent mage armor, shield, protection from evil etc for 100 XP each. Similarly, the 2nd level buffing spells like Bull's Strength would be cheap for 600 XP. Finally, Greater Invisibility at 1500 XP sounds like an awesome deal for a rogue, unless he intended to keep having a life as a humanoid. ;)

The problem is probably that the caster must pay the XP price rather than the recipient, so it's hard (or expensive) to get wizards to cast it on you. Paying the price for a casting of TP and 3000 gp for the XP ends up more expensive than an item that gives +4 to Strength, right?

What do you mean with "any spell currently in effect"? In the world? Cast by the TP-caster? In effect on the TP-caster?