Koh Samui is one of the more expensive places in Thailand to get around. If you do not want to be overcharged you need to make sure:
1. You agree a price before you get in the taxi.
2. Negotiate (without getting into an argument) and be prepared to walk away if you cannot agree a price.
Private taxis in Koh Samui are meant to run on the meter. None of the taxis we took on our last visit in February 2016 did. Shared pick up truck taxi (‘songthaew’) are meant to use fixed fares. We found that the same journey has cost different amount on different occasions. The underlying problem appears to be one of a lack of competition on the island. Different groups of drivers appear largely to have exclusive rights to run taxi in particular areas or on particular routes. This means they do not have to worry about competitors under cutting them.

The Thai authorities are aware of the problem, and overcharging for local transport is seen by the Thai Government (quite correctly) as an impediment to the development of tourism in Thailand – firstly, it puts people off coming back and, secondly, they tell their friends back home and they decide not to visit because they do not want the aggravation. Thailand’s premier, PM Prayuth Chan-Ocha, has recently announced a crackdown on overcharging by local transport providers in 6 tourist destinations, including Koh Samui.

In the meantime, until the Thai authorities impose effective regulation, let me tell you about how much my Thai wife and I paid for our taxi trips around Chaweng by negotiating the price. For anyone visiting Koh Samui we suggest that they be very careful as to how they approach the business of negotiating. Taxi drivers can become violent if they feel someone is being disrespectful and on occasion they (sometimes not alone) will attempt to intimate visitors with violence to get them to pay extortionate prices. This is not hearsay. We see it every time we go to Koh Samui.
To avoid these problems, whilst getting the best price, negotiate before you enter the taxi (not after) and simply walk away saying ‘no thank you’ if the price is not right and wait for another taxi. Taxis, private and shared, will give lower prices to people if they negotiate well.
We did not argue with people, but simply said no thanks and walked away, and these are prices we were quoted and the ones we actually got:
Private Taxi from Tesco Lotus Chaweng to Central Festival Samui (10 minutes)
Quoted: 300 THB
Negotiated price: 200 THB
Private Taxi from Central Festival to Tesco Lotus Chaweng (10 minutes)
Quoted: 400 THB
Negotiated price: 200 THB
Shared Taxi from Tesco Lotus Chaweng to Nathon (30 minutes)
Quoted: 100 THB per person
Negotiated price: 60 THB per person
It is probably possible to get cheaper prices if you really know the place and taxi drivers see you regularly. For most visitors the prices we paid are likely to be the cheapest that can be negotiated. Do not forget it was a Thai visitor (my wife) who did the negotiating and you may find that as a foreigner you simply cannot get the lower price.