The only likely minable resource in space with realistic economic potential is Life itself. If there is life in the solar system, and if it is based on a different biochemistry than ours, the microbes in a cup of Martian soil or the mist from a geyser on Europa might be invaluable, even if only recoverable in tiny quantities.

An entire planetoid of high-grade nickel-iron, or a vein of rare earths might be too expensive to recover for terrestrial industry to drag home and exploit successfully, but some alien bugs would be the key to an entire new biochem or genetics industry. Even fossils of alien life might be of great value: consider that petroleum is a fossil, and it can be refined into all sorts of useful products. Complex molecules can be altered and reassembled into new compounds, and the products of an alien biochemistry might have totally unexpected properties. We wouldn't even need to bring them home in bulk quantities, if we found something really interesting we could always synthesize it here on earth.

The only thing I can think of more value than an alien biochemistry would be an artifact from an alien civilization, perhaps a wrecked probe or derelict spacecraft from another stellar system. But that's a long shot. Space germs are a lot more likely and a distinct possibility.