I say the following knowing that everyone has their places they like to save or spend money on a trip and I have no idea of the overall context of the OPs visit. But whenever I did BKK-HuaHin I would always suck it up and pay for the hotel limo (usually a Benz or 7 series). The traffic can be terrible and it can wind up being a long trip to be in the back of a taxicab or on a bus. The times we've done this we have been on our way to a relaxing vacation and the idea of starting it in the limo was a good one. It wasnt cheap, that is for sure, but it was worth it and we just ate some less expensive meals and did other things to try to offset that cost a bit. Since we were always returning to Singapore, I can second/third that you really do have to drive back to BKK. And depending on your flight time, it can take much longer getting back. As a result, Hua Hin isnt really a convenient beach stop for a visitor to Thailand, Phuket or even Pattaya (since the airport is on that side of the city) are more convenient.