Update: As noted in the comments, the price changes regularly. Keep updated, because I’ve never seen it go as high as Microsoft’s official pen.
Microsoft’s Surface Pro 3 has excellent inking via its N-Trig active digitizer. The included official pen, however, is a bit expensive at $49.99 from the Microsoft Store. It’s not something you’d want to casually lose, nor is it an impulse buy as a backup.
Fortunately, there’s an alternative. The HTC Scribe pen (click for technical specs), intended for products like the EVO View tablet, works just fine with the Surface Pro 3, supports pressure-sensitivity, and has the same two-button support as the official pen. It’s available right now from Amazon for $11.99, and it’s eligible for Prime if you’re so inclined. An excellent deal, and highly recommended.