The amount charged is different this month than it was last month.
「請求金額が先月と違う」は次のように言えますよ。
ーThe amount charged is different this month than it was last month.
amount charged で「請求金額」
例:
The bill is always the same amount every month, but it's less this month for some reason. Did you forget to charge me for 〇〇?
「請求書はいつも毎月同じ金額なのに、なぜか今月は少ないです。〇〇の分を請求するのを忘れましたか?」
ご参考まで!