The modified code which is tuned to request data from your own server instead of coin market cap directly

crypto-prices.js

function get_curr_coinmarketcap_price(crypto_id, unused_param) {

  // note that the unused_param is to to force the function to rerun
  //   and recalcuate the functions results when the function is called with 
  //   new input
  // https://stackoverflow.com/questions/17341399/refresh-data-retrieved-by-a-custom-function-in-google-spreadsheet

  // create a string that is the first part of our API calls url
  var url1 = 'https://yourserver.com/crypto-prices.php?crypto_id=';

  // append crypto_id to our url so we can tell the API that we want prices 
  //   only for that id
  var full_url = url1 + crypto_id +'/';

  var response = UrlFetchApp.fetch(full_url);

  //return response.getContentText();
  // parse the JSON string and build it into JavaScript objects
  var data = JSON.parse(response.getContentText());

  // return the first (should be only) price from data
  // extract the value in the dictionary with a key of 'price_usd'
  return data[0]['price_usd'];
}