Summarising discussion:
(1) Plural/singular verb agreement with units: ect's answer at this related question includes
- Units of measure are treated as collective nouns [in the US usually are], taking a singular verb.
• For each patient, 10 mL of whole blood was collected in a clot
tube. [Penn State: Effective technical Writing]
Here, notional agreement (10 ml being a single quantity / sample) is accepted on both sides of the Atlantic.
- $10 / £9 seems a reasonable price.
- 20 meters / 20 metres is too long. I'll use the 5m cable.
(2) This notional agreement extends to the 'how many ...?' questions, though less strictly – doubtless because of the pull of proximity agreement as a singular verb form may sound distinctly jarring. Raw Google results for "How many grams of flour is" outweigh those for "How many grams of flour are" by more than 3 : 1. 'How many grams of flour are 100ml?', with a parallel measure as complement (arguably subject) sounds awful. ('How many grams of flour are equal to / correspond to / are (or is) the same as 100ml?' sound far more natural.) And '*How many miles are it to Jerusalem?' is unacceptable (though 'How many miles are left?' sounds far more natural).
So, there is divided usage, and each case must be considered separately. If considered clumsy-sounding (or of course outlandish), rephrase ('How much flour is [there] / What is the weight of the flour on the scale?', etc).