Surf boards are available to be rented in almost all the surf locations. The prices are per hour and change slightly for locals and foreigners. To provide an exact figure its about Rs.250 (roughly $1) per hour in most of the surf points (eg: Weligama, Hiriketiya and Unawatuna). You can also rent a board for a full day and will be a cheaper option if you are planning to surf for couple of hours.
But it should be noted that the prices in the East coast are comparatively higher to the South coast (not sure what the reason is!).
Note – As of 2022, the prices of renting a board are changed along with the depreciation of the Rupee against the Dollar. But roughly renting a board would cost about $1 per hour