So in the United States, one acre of wheat yields an average 37.1 bushels of wheat. That's about 210,000 kcals of flour per acre using modern machinery.

If we look at https://livinghistoryfarms.wordpress.com/2015/07/28/grain-ha... we find that in the 19th century a skilled cradler could harvest 1 ½ -2 acres a day.

Grain was harvest by hand until the first century when the vallus was invented so I'm going to just estimate a difficulty increase of 4 people being able to harvest an acre in a day without a vallus or a cradle.

Beloch's 1886 estimate for the population of the empire during the reign of Augustus suggests there were 23 million European Romans, if we assume 'Rome' means the city I haven't a clue what the population would have been, if we assume the whole empire then Beloch's estimate was 54 million.

Let's just assume 1 million people with 20% of their caloric intake being grain and assume an even 2000 kcals per person.

So we need a bit over 1900 acres assuming no spoilage per million people. Wheat has a fairly narrow harvest window as you need to harvest it once it reaches a certain moisture level and if you leave it too dry too long it can easily be damaged/detached from wind and rain.

Let's assume a team of 4 people can realistically harvest, and prepare for further drying, 5 acres in the window.

That means we need 3800 people per million mouths fed a diet that gets 20% of it's kcals from wheat using ancient harvesting methods. Obviously this is an estimate but I tried to be generous in favor of the harvesting and my kcal number is likely low. Yields would also be markedly reduced back then without chemical fertilizer/herbicides/pesticides.

So 3800 people per million mouths earning 800-1000 sesterces a year ( https://www.slideshare.net/javierredondas/jobs-salariesrome ) let's assume another 10% in cost for milling (probably low) so we'll assume 3,762,000 sesterces per 1 million people for 20% of their kcal needs.

Converting sesterces to modern money is a bit tricky but at 1990 silver prices a single sesterce was worth about $1 and by mid 2010 would have been about $3.25 according to https://www.globalsecurity.org/military/world/spqr/money-1.h...

So anywhere from 3-12 million dollars per million people. I'm guessing this would have been more like 15-45 million dollars per million people so even with that figure we're looking at maybe 2 billion dollars for all of the Roman empire.

So yeah, I like the (IIRC) 1.5 billion dollar figure mentioned in the book I referenced elsewhere which is why I left it at 1 billion and then some in my comment.