"What is the sum?"
Translation:Ποιο είναι το άθροισμα;
I guess you could say Πόσο είναι το ποσό;" - What is the sum?
"Τι είναι το ποσό" may be a direct translation, but it doesn't quite make sense. It's almost like you're asking about the quality, when the sum is clearly about quantity ^.^
What is, in some cases, used instead of how much. For example:
What's your income?
What's your sum total?
What's the price of this cup?
I hope I helped a bit. ^.^