Yodobashi net shopping price watcher by Google Apps Script. Please see https://qiita.com/xtetsuji/items/953da9e71eb28741804c for detail.

yodobashi-price-watch.gs

var mailto = "YOUR_PHONE_ADDRESS";

// 定期的に呼ばれる
function walkWritePrice () {
  var spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = spreadsheet.getSheetByName("価格ウォッチ");
  if ( !sheet ) return;
  var dateCell = _dateInsertionCell(sheet);
  dateCell.setValue(today());
  var urlColumnNumber = 2;
  var inputCell, url;
  for ( var i = 2 ; (url = sheet.getRange(urlColumnNumber, i).getValue()).length > 0 ; i++) {
    if ( !url.match(/^https?:/) ) continue;
    if ( i > 1000 ) break; // for safe
    inputCell = sheet.getRange(dateCell.getRow(), i );
    inputCell.setValue(yodobashiPrice(url));
    coloringCell(sheet, inputCell);
  }
}

// _dataInsertionCell: sheet -> cell ; 日時を入れるセルを割り出す
function _dateInsertionCell (sheet) {
  var i, cell;
  for ( i = 1 ; i < 1000 ; i++ ) {
    cell = sheet.getRange(i, 1);
    if ( cell.getValue().length === 0 ) break;
  }
  return cell;
}

function coloringCell (sheet, cell) {
  // 一つ上のセルより価格が安かったら黄色にする
  var upperCell = sheet.getRange(cell.getRow() - 1, cell.getColumn());
  if (!upperCell) return;
  var upprice  = parseInt(upperCell.getValue());
  var curprice = parseInt(cell.getValue());
  var subject, body, kataban = sheet.getRange(1, cell.getColumn()).getValue();
  if ( upprice > curprice ) {
    cell.setBackground("yellow"); // 値下がり
    subject = "値下がりレポート";
    body = kataban + "が"+upprice.toString()+"円から"+curprice.toString()+"円に値下がりしました";
  } else if ( upprice < curprice ) {
    cell.setBackground("red"); // 値上がり 
    subject = "値上がりレポート";
    body = kataban + "が"+upprice.toString()+"円から"+curprice.toString()+"円に値上がりしました";
  }
  if ( mailto && subject && body ) {
    MailApp.sendEmail(mailto, subject, body) ;
  }
}

function pad (arg) {
  var s = arg.toString();
  return s.length == 1 ? "0" + s : s;
}

function today() {
  var now = new Date();
  var string = now.getFullYear().toString() + "-" + pad(now.getMonth()+1) + "-" + pad(now.getDate()) +
               " " + pad(now.getHours()) + ":" + pad(now.getMinutes());
  return string;
}

// URL の価格を調べる
function yodobashiPrice (url) {
  var res = UrlFetchApp.fetch(url);
  var price, m;
  if ( res.getResponseCode() == 200 ) {
    m = res.getContentText().match(/id="js_scl_unitPrice".*?([\d,]+)/);
    price = m ? m[1].replace(/,/, "") : -1;
  } else {
    // TODO: 別の方法で知らせたほうがいいかも
    Logger.log("URL="+url+" fetch error.");
  }
  return price;
}