16 hours at CMB is not worth it. Depending on the time of the day you better get a hotel near Negumbo and go to the beach or go to Colombo city. The fare should be around 800 to 900 SLR, which I think is 1USD is about 120 SLR or so. This is one way. There is a transit hotel counter once you pass customs and get out. You can book a hotel there too and they would organise the transport etc.