price-per-area.js

function pricePerArea (area) {
  var areas = [0.15, 0.5, 0.75, 1, 5, 10, 15, 20, 35, 50, 75, 100, 150, 200, 350, 500, 750, 1000, 1250, 1500, 2000, 2500, 3000]
  var prices = [8, 8, 5, 4.5, 4, 3.8, 2.8, 2.6, 2.4, 2.2, 2, 1.6, 1.4, 1.2, 0.9, 0.75, 0.65, 0.5, 0.4, 0.35, 0.25, 0.15, 0.12]

  // catch edge cases
  if (area <= areas[0]) {
    return prices[0]
  }
  if (area >= areas.slice(-1)[0]) {
    return prices.slice(-1)[0]
  }
  // find the index of lower area value
  var i = areas.findIndex(function (a, i) {
    return area > a && area <= areas[i + 1]
  })
  // calculate the gradient
  var a = (prices[i + 1] - prices[i]) / (areas[i + 1] - areas[i])
  // calculate the offset
  var b = -a * areas[i] + prices[i]
  // interpolate
  return a * area + b
}