Excellent question! 

Judging from the WOIAF, we do have some crude systems of regional redistribution of grain in times of crisis, as seen by the fact that Aegon V sent grain up to the North during a particularly bad winter. I would argue that the North’s intense attachment to guest right (and its less frequently mentioned tradition of self-euthanasia during long winters) has a lot to do with a sort of crude welfare state of seeking food and shelter at Winterfell. (We also see the Starks exercising very close control over food reserves, for example)

But in terms of a regular grain dole, we actually have an example from Westerosi history:

“Ultimately, some have wondered if the king’s near death in Dorne did not affect his mind in some way, for as the years of his reign progressed, his decisions grew ever more zealous and erratic. Though the smallfolk loved him—he emptied the treasury regularly to fund his charitable acts, including the year when he donated a loaf of bread daily to every man and woman in the city—the lords of the realm were beginning to grow uneasy.” e

Ultimately, some have wondered if the king’s near death in Dorne did not affect his mind in some way, for as the years of his reign progressed, his decisions grew ever more zealous and erratic. Though the smallfolk loved him—he emptied the treasury regularly to fund his charitable acts, including the year when he donated a loaf of bread daily to every man and woman in the city—the lords of the realm were beginning to grow uneasy.

So Baelor the Blessed supposedly bankrupted the monarchy by (among other things) providing a bread dole for the population of King’s Landing. Now, according to AGOT, a tart costs around three coppers, which I’ve been using as a pre-war price for a loaf of bread. That would suggest that it would cost around 127 dragons a day to buy everyone in King’s Landing a loaf of bread, or 46,355 gold a year. 

At that rate, a grain dole for the whole of Westeros would cost 3.7 million gold a year, or 0.7% of GDI, or 7% of total tax revenue. This seems surprisingly affordable, although based on my old estimates of royal income, it would bankrupt the monarchy. 

However, you have to keep two things in mind: first, it’s all based on the estimate of how much bread costs. If bread costs more than 3 coppers a loaf as I had originally estimated, the price skyrockets. Second, I could be quite wrong about GDI and thus tax revenue.

So let me see if I can approach it from another angle. I’ve estimated that the average yearly income is between 3-5 gold. During the Ancien Regime, the average worker spent about half their income on bread, although this could spike as high as 88% during crop failures. (That suggests that the average Westerosi spends 48 coppers (or ~1 silver) to 80 coppers (or 1.5 silver) a day on bread.) 

In turn, this would suggest that a grain dole would cost 60 to 100 million dragons a year. That’s 11.4 to 19% of GDI, or 114% to 190% of total tax revenue.

So I’m going to go out on a limb and say that either three coppers is the wrong price for a loaf of bread, or the smallfolk of Westeros eat 16 tarts a day, which seems unhealthy.