The train option

A few sharp intakes of breath at the prospect of paying almost 7000 Baht to get to and from Ubon yesterday. So I've been looking at the possibility of doing at least part of the journey by train and getting up there works out quite well with an overnight train on Tuesday evening leaving Don Muang at 21:00 arriving early on Wednesday morning, A first class sleeping berth would be about 1200 Baht but a second class a/c is only 691/761 Baht (upper/lower berth) which could shave up to 1700 Baht off the cost.

Coming back by train is also possible but the train only has 2nd class a/c seats which are 551 Baht and would involve a 10 hour plus journey on Sunday slated to arrive at Don Muang on paper in good time to  connect with either a Thai Lion flight at 19:50 (575 Baht) or the last flight on a Sunday to Hatyai at 20:00 (Air Asia: 1340 Baht). So the cheapest option is as follows:

Tues. April 12, HDY - BKK, Air Asia, 17:40 -19:05; BKK - UBL, Thai Rail, 21:00 - 07:30 (Wed)
Sun. April 16, UBL - BKK, Thai Rail, 07:30 - 17:38, BKK - HDY, Lion Air, 19:50 - 21:10

This works out around 3000 Baht depending on your choice of bunk position on the train up, with a 1st class berth  adding another 500 Baht and the Air Asia flight instead of Lion Air on Sunday adding another 750 Baht.

Food for thought.