you should just stick to one guesthouse only, as all the attractions are easily reachable by public transport especially the mtr.  You can consider moving to different guesthouse if you go out of seoul or the 1 way journey already takes more than 3 hours.

2 months ago, I went to jeju from seoul for RM112, and from jeju back to seoul for RM94 per person (net price, after includes tax, currency conversion, etc). I booked the tickets 3 to 4 months earlier. I booked the ticket when they released the fare schedule for the first time, like in the first week.