You can try YEHAtour (http://www.yehatour.com/jeju2014/tour/yehabus/main.php?lang=eng&lcode=01). During matta last matta fair, YEHA has their own booth to promote their jeju package and their guesthouse..  Next month, there'll be another MATTA FAIR at pwtc. You may come to their booth and compare with other travel agents as well.

Other than bus tour or rent a car, you can also consider rent a taxi..  by renting a taxi, you can select yourself on which attractions you want to visit.

based on the link i give you above, bus tour cost 78k won per person. If renting a taxi, cost is about 120K won per taxi, that can fits up to 4 adult.

18k just good enough to cover entrance fees for nature's attraction.. like waterfall, ilchubong peak, yeongmeori coast, cliffs, odeolgae rock, etc, caves, etc.  When it comes to muzium and other man made's stuff, it becomes expensive.. loveland ~18k, seongup folk village ~16k, etc. And you have to cover the taxi driver's meal too.. meal in jeju is costly. it's around rm30-rm50 per meal set..

I don't want to say taxi is better than tour bus or vice versa. I'm just giving you an opinion and idea. You need to plan the detail yourself and see which gives better savings..