I ask myself, OK, this guy is a 4-of in any deck that wants to play him. He makes tokens with Prowess for the low cost of casting any non-creature spell in the history of ever. The Mentor himself also has Prowess. I sit and ponder how this guy isn't running over Modern yet. We know how good Abbot of Keral Keep has proven to be. Monastery Swiftspear is one of the best one-drop creatures in Magic the Gathering history. Prowess is really, really good.
Why is he only $15? That doesn't really sound cheap, and frankly it isn't. But honestly, it's not like the Mentor is dominating Standard right now. He was a $25 card for awhile, but then he hit a low of $14 and is only creeping up slowly because some players realized that playsets of this guy should be at least $100 come rotation. That's probably conservative.
Here's what really gets me though. Buylists are pretty bullish on this guy, usually paying $10-12 a copy and no one is selling. This means those people that have them want to keep them. When you have 10-20% spreads between buylist and retail on a Standard playable card, especially one that's a mythic rare strong in a Eternal formats, that usually screams price jump. So if you don't have a playset of Monastery Mentor, go get your copies on PucaTrade or at your local game store. They're a lot easier to get than they probably should be. You'll be glad you got them.