Some Hotels, like Amari does not assist you with airport transfer unless you are taking their rather pricey limo service. The one in Amari cost 900b.
For 500b from any tour agents or taxi providers, you could enjoy a similar service. Just walk up to them the day before you're leaving and pay them.
I'm not sure if this is the cheapest, but definitely it was hassle free and really comfortable. I booked mine from Jenny Tours.
Taking into consideration the distance and gas prices, I think it is a fair price.