var obj = {
  online_rate_1: "$5.00",
  online_rate_2: "$22.00",
  online_rate_3: "$44.00",
  rate_1: "$9.00",
  rate_2: "$18.00",
  rate_3: "$30.00",
  rate_description: "<p>Some description text</p>",
  time_period_1: "0 - 0.5 Hours",
  time_period_2: "0.5 - 1 Hours",
  time_period_3: "1 - 2 Hours",
}

function findKeysWithPrefix(object, prefix) {
  var arr = [];
  for ( var property in object) {
    if (
      object.hasOwnProperty(property)
      && property.toString().startsWith(prefix)
      && /\d/.test(property.toString())
    ) {
      arr.push(object[property]);
    }
  }
  return arr;
}

console.log( findKeysWithPrefix( obj, 'online' ) );