I would recommend the Bill Resort in Lamai. It's conveniently located, located on a nice bit of beach and the service is excellent. Bringing a guest back isn't a problem either.
It's around 15 euro for a Hut with hot water, lovely bed and satelite tv.
Here's the link if you're interested...
http://r24.org/phuket-info.com/samui/billresort/