"Have you" is perfectly fine English when asking a question. For the record, I'm originally from the USA
so TU ai o sumă mare is also perfectly acceptable?
Yes. Just like in Italian and Spanish, however, you just omit it unless you want to emphasise the subject. Using it is not incorrect per se, yet it will sound strange in most cases.
In English, we say the grand total, which seems to also be a good translation, but not accepted.
I would think if this phase were ever to be used it would more likely mean 'Are you very rich' but my knowledge of Romanian is very limitted.
I don't understand how "grand total" would fit here. It's asking if they have a lot.
Re: Gzeebzee comments, a "large sum" may not be the actual "grand total". And, "Are you very rich?" = "Tu ai/Voi sunteți foarte bogat?"
Sounds like duolingo is a gold digger