Do compare prices you get with Bangkok Airways, easily the best internal airline in Thailand, and worth a few pence more.
Book ahead as January is peak time, and flights will be full.
Alternatively, travel the slow way. Relax and enjoy the scenery, meet the locals, take the train!
http://www.seat61.com/Thailand.htm#Krabi
No station in Krabi, but the link has details of train / bus combinations.
If you're travelling independently, it's much better seeing the country from ground level as opposed to 30,000 feet!