zmienKolor()

function zmienKolor() {
    var priceLimit = parseInt(document.getElementById("check1").value);
    changeBoxColorAccordingToPrice(priceLimit, 1, "box1");
    changeBoxColorAccordingToPrice(priceLimit, 3, "box2");
    // ...
}

function changeBoxColorAccordingToPrice(priceLimit, priceElementIndex, boxId) {
    var price = document.getElementsByTagName("h3")[priceElementIndex].innerHTML;
    // ....
    if (priceLimit >= price) {
        document.getElementById(boxId) = // ...
    }
}