勾選時激活input，不然禁用

window.onload = function(){
    var price = document.getElementById('price');
    price.disabled = true;
    price.style.padding = '2px 3px';
    price.style.background = '#eee';
    price.style.border = '1px solid #ccc';
    var tj = document.getElementById('tj');
    tj.onclick = function(){
        if(tj.checked == true){ //可編輯
            price.disabled = false;
            price.style.background = '#fff';
            price.style.border = '2px solid #ff7d00';
            price.style.padding = '2px 3px';
            price.focus();
            
        } else {
            price.disabled = true;
            price.style.background = '#eee';
            price.style.border = '1px solid #ccc';
            price.style.padding = '2px 3px';
        }
    }
    
}

<form action="#">
價格：<input id="price" type="text"> <input id="tj" hidefocus=true type="checkbox">特價
</form>