What do you consider reasonable? Kurokawa tends to be fairly pricey, but it's also considered one of Japan's nicest onsen. You're looking at a minimum of about ¥12,500 per person for an overnight there. There are some ryokan listed at the
Japanese Guest Houses site (although without exact pricing details) and more listed at the
Kurokawa Onsen site (but only in Japanese). Sanga Ryokan seems to be a good bet - according to the latter site, they start at ¥12,600. Okyakuya Ryokan doesn't have pricing shown on that site, but according to a travel guide I have, they have rooms from ¥12,500 a head.
According to the Kyushu Sanko Bus website, there are buses leaving Aso Station at 10:09, 11:19, 14:03 and 15:59, taking roughly 50 minutes to get there. The timetable is current as of October 2008, so you may want to double-check with tourist information in Aso or Kumamoto.