The cheapest way to get into Colombo from the airport, is by Train and is the least known way to travel cheaply roughly a 3rd of the cost of a bus.
They are 11 trains so check the timetable first before you leave the airport.
Take the free Airport shuttle to the bus station.
Then walk 1km, if you don not have luggage, by following the road (Canada Friendship) to the Katunayaka South station.
If you have luggage, best just to take a Tuk Tuk if not metered haggle.
Buy a ticket to Colombo Fort, you will arrive right next to where the buses drop people off.
Next cheapest is air-conditioned public bus the number 187-E3, and goes to Colombo central bus station from the airport, approx 30 minutes as it takes the expressway to Colombo. 2 buses run every hour last bus from Colombo is 7:30pm and from the Airport is approx 8pm they start again at approx 6am from Colombo.
There is limited luggage storage space at the rear so be prepared to have your luggage on your lap. Do not accept help from anyone, if you do put your luggage in the rear wait till the driver secures you luggage. There is no charge to store your luggage in the rear.
After 7-8pm your only option is the private buses, they cost a little more but run all night. These take about an 1 hour as they do pick up locals along the way and can get packed. Do not take up a seat with your bag as you will be charged.
Note: Bus conductors will try it on, if you think the cost is to much tell him no and start to walk off the bus. On a private bus (most expensive) you should not pay more than RS150 (Feb 2017).