ok. haven't got all the details down, 'cause i've been piecing it all together from different forums. but i think it can potentially be done in 24 hours, although maybe two days. the cost of the trip would probably be about 500RMB per person, i estimate. train to erlian is about 130 each way and then have to grab a transport to the border for a negotiable price, then go back to erlian. might have to stay overnight, though i'm not so sure. big hold up is if you are not american you have to spring for a mongolian visa, and that sets you back 500RMB or so.

going to hk is obviously much more straightforward, but it's also more expensive, and kind of boring. i've been to hk/shenzhen enough times already. mongolia is more of an adventure.