An electronic notice board, measuring 4 m × 3 m, is mounted on a square wall with sides 6 m × 6 m. Pablo must paint all of the wall not covered by the notice board. A litre of paint covers 3 sq. meters of wall. Paint can be bought in 1 litre cans costing $10 each or 2.5 litre cans costing $20 each. What is the least that Pablo can spend on paint that allows him to complete his painting job?