There are basically 2 different kinds of bananas that are sold in Thailand. It’s the small domestic one and the curved normal banana. You can buy them on the street, markets and off course in the shopping malls. Most big markets sell them for around 25-60 THB for a bunch of 15 bananas and the shopping mall price is never less then 50 THB. If you buy the bananas yellow you need to be sure that you are going to eat them within a day or two because they age very quickly. If you buy the green once it can take few days for them to get yellow and they will get to old to quick.
So if your household is only one person go for the smaller package around 20 THB for 5 bananas. The smaller bananas go for around 30 THB for 20 of them and they have the same problem but not same good taste.