Be that your best price?与 Is that your best price?即用be与is的区别

Be that your best price?与 Is that your best price?即用be与is的区别
请问各位过路神仙,有的场合会用原型动词be,而有的场合用am、is、are,有什么区别吗?
我知道在情态动词后会用原型动词be,但上面的句子没情态动词啊,怎么也可以用原型动词be?
请给予讲解,为谢!