A small manufacturing company produces two models of a surfboard: a standard model and a competition model. If the standard model is produced at a variable cost of $210 each and the competition model at a variable cost of $300 each, and if the total fixed costs per month are $6,000, then the monthly cost function is given by

C(x, y) = 6,000 + 210x + 300y

where x and y are the numbers of standard and competition models produced per month, respectively. Find C(20, 10), C(50, 5), and C(30, 30).