If you go in person to a few hotels in Kenting Street to get information about their rooms, you'll find better prices than what you can find online.
So, unless you are visiting at a very busy time, it is best to book only a night or two online in advance. Then go to a few hotels one by one to ask for the price, see the room (take also a few second to test the wifi if it matters to you). You can always try to bargain, especially if you are staying for a few nights.
Prices on the weekend are usually around 20% higher than during weekdays.
If you're worried about the noise from traffic or the night market, see if you can get a room that doesn't face the street directly. The night market begins at 6:00 pm and ends at 10:00 pm. Bars will usually lower the music after 10:00 pm and don't seem to play anymore after midnight.
8 days / 7 nights at the Surfer Beach Camp (Weligama, Sri Lanka) situated just in front of a great beginner breaks that will get you up and ripping in no time, If you have in mind to have the perfect beach holiday and the waves breaking just meters away, this is it.