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>