I'll pay whatever it costs for my son and I at a later day.
I'll pay you the total for my son and I afterwards.
この場合、次のような言い方が自然で良いかと思います。
ーI'll pay whatever it costs for my son and I at a later day.
「息子と私の分は後日全額払います」
to pay whatever it costs で「(費用が)かかる分だけ払う」
ーI'll pay you the total for my son and I afterwards.
「後ほど息子と私の分の合計を支払います」
to pay the total で「総額を支払う」
ご参考まで!