What is reasonable expenditure on meals during a conference?
Ask your advisor. Your university is likely to have a maximum allowable per diem that is based on the city that the conference is in. It varies from university to university, but mine just pays the per diem for that city regardless of how much you actually spend. Many universities do it this way. It saves time and money processing expenses on a meal-by-meal and receipt-by-receipt basis. If your university does this, then it doesn't matter how much you spend on yourself.

Also, given that the per diem is likely capped in the $40-$50/day range anyway, you are unlikely to blow your advisor's travel budget on food even if you hit the max every day.