MP4 Nation offers the FiiO E11K Kilimanjaro2 Portable Headphone Amplifier for $69.50. Coupon code "E1120OFF" cuts it to $49.50. With free shipping, that's the lowest total price we could find by $10, although we saw it for $6 less in February. It features an aluminum-alloy body, ALPS potentiometer, OPA1642 op-amp, and the ability to drive headphones between 16 and 150 Ohms.
Note: It ships from Hong Kong and should arrive in two to four weeks.