Vividcard has a review on Head-Fi of the Fiio Q1 portable USB DAC/AMP with a going price around $70. The price is great for an entry level DAC/Amp with a tested battery life of 24 hours.
Summary: “The Q1 is a great purchase for an first time amp buyer. It allows a large feature set at a very low cost. And as FiiO has always managed to do, they have provided a product that is not only low cost, but has a big sound. Overall I am very happy with my purchase as it has allowed my sound quality to improve in all areas. And having the extra power to drive more difficult headphones is an added benefit of future-proofing myself. The battery life is also large enough be able to use the device for a very long time without having to search around for a charger. In short, another fantastic product from a reliable and consistent company!”