// background.js
chrome.runtime.onMessage.addListener(
    function(request, sender, sendResponse) {
      if (request.contentScriptQuery == "queryPrice") {
        var url = "https://pixelboii.wtf/steamvalue.json";
        var skinFullName = request.itemURL;
        fetch(url)
            .then(res => res.json())
            .then(price, priceNumber => {
              sendResponse(price.items_list[skinFullName].price["7_days"].average + price.request.priceNumber);
            })
            .catch(err => { throw err });
        return true;  // Will respond asynchronously.
      }
    });

// content.js
function getPrices() {
    for (var i = 0; i < 35; i++) {
        chrome.runtime.sendMessage(
            {contentScriptQuery: "queryPrice", itemURL: skins[priceNumber]},
            price, priceNumber => {
                console.log(price + priceNumber);
            });

        priceNumber = priceNumber + 1;
    }
...
}