The cost of two shuttle flights.....
That's about what I figured. It really doesn't take much technologywise. "Docking" with a NEA is actually less risky and can require less delta-V than lunar orbital insertion maneuvers, lunar descent/landing/ascent stages etc. Yeah, there are some "obstacles" like radiation shielding, but they're hardly insurmountable. We just need a suitable target now (maybe 1998 KY26 like someone has suggested) - and then a completely costed reference mission can be formulated in minute detail. If the cost is $900m or under, you never know who might take an interest. You just never know. :-)