Colombo Airport to Polonnaruwa is about 200 km.
The best way is by taxi. Based on LKR 60/km for one-way hires that should be around LKR 12,000.
By bus you will have to take a free shuttle from the airport to Katunayake bus stand. From there bus 5/245 to Kurunegala and then bus 48-8 to Polonnaruwa. Buses are dirt cheap but you will have problems with large luggage - maximum is a large backpack.
By train it's best to go from airport to Colombo Fort (bus 187) and catch the train. There are only 3 trains daily. One-way LKR 340/2nd class.
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