The main problem is that you only wrote the Japanese translations, and didn't provide original English sentences (more accurately to say, your original ones might be Korean sentences, though).
Your initial Japanese sentence 塾の授業料より課外費用が高いので、ちょっと負担になります。 is interpreted that the speaker 幸子 is talking about her 負担 of 塾. That's why I used それ以外の費用 because she already said that her daughter ダウン only uses 塾. If she wants to say that she doesn't use private lesson because the fee is higher than 塾's fee, you need to add から at the end of her words.
By the way, I don't think 課外授業 works well there. Anyway, first of all, you should unify the Japanese translations of each term.
revised version
幸子:うちはどっちも塾に通っています。 個人レッスンは費用がかかるから。(かかりますから sounds too polite, therefore stiff/distant.)