in half a price还是 at half a price?

in half a price还是 at half a price? at half a price/at half price/at a half price哪个是对的哪个是错的...为什 半价用英语是at a half price 还是没有a 是at a low price 还是in a low price only students can get it at half price. 同义句 you can get it a 3 half-price 价格合理是at a good price还是at good price at a good price 还是 for a good price at a cheaper price 和in a cheaper price 英文中一般用pay half the price 还是用offer half the price 为什么? at a good price rationalization at a price