They'll tell you.How much do you have to pay?（合为含有宾语从句的主从复合句）
问题描述：
They'll tell you.How much do you have to pay?
（合为含有宾语从句的主从复合句）
答
They will tell you that how much you have to pay.that 引导的宾语从句,第二个句子做宾语从句,由疑问句变陈述句,去掉助动词do .