The economics of the game are profoundly broken. Just roll with them.
The CRB says 10gp/month for average lifestyle but it seems very wrong... 3gp for a month of average meals, so a rent for 7gp ?!!
I think this is one of your issues. Food and rent are both substantial parts of cost of living, but they're not the only thing.
Modern financial planners suggest that you should spend no more than 25% or so of your monthly budget on housing, and a similar amount on food. This leaves utilities (e.g. firewood), clothing, entertainment, savings, and such like. for the other 50%, so 3 gp for housing for a month seems reasonable and roughly inline with your earlier assessment.
An expert level 1, with +5 profession will earn (4x1d20+5)/2 gp each month, on average it's 30gp.
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With peasants earning 300gp [n.b. 30 gp, not 300 gp]a month?
The other thing to note is that "experts" are generally highly skilled, almost by definition. I'm not sure I would describe your expert level 1 as a "peasant." I'm also not sure I would assume that a peasant-commoner necessarily has a positive stat modifier to his day job skill, that his day job involves a class skill for him, or even that he has a day job skill. "Unskilled labor" is a thing, even today.
So, yes, if you have a moderate level of skill, it's fairly easy to maintain an "average" lifestyle. But untrained characters make only 1sp per day, roughly 3 gp per month, which means that even a "poor" lifestyle leaves them with nothing at the end of the month.
I tried something, tell me if it feels right.
Let's say for a commoner, 1 sp = 1$ from now.
So 1gp = 10sp = 10$
An expert level 1, with +5 profession will earn (4x1d20+5)/2 gp each month, on average it's 30gp
300$/month, seems OK for now, it's not modern world heh.For the costs of buying a house, we'll refer to the ultimate campaign book, and an average house in town seems to be 1290gp = 12900sp = 12900$ (again, it's not modern world and modern prices...seems ok..).
By the way, I generally go with the "1 cp == $1 modern USD" equivalence instead.
That, of course, means that our level 1 expert will make about $3000/mo or $36,000 per year, which is in the ballpark (median personal income in the US for people over age 25 was $32,140 in 2005), and that an average house is $129,000 (median house price in 2015 was about $190,000; again, that's in the ballpark.)
Median rent is about $800, which is about 1/240 of the median house price -- but we've also seen a drop in house prices and a rise in rent recently, so that's higher than the long-term average. 1/333, again, is about in the right ballpark.
So you actually get fairly sensible numbers against the modern world if you use the $1 == 1 cp conversion factor.
It even works out fairly sensibly for cost of living. Average cost of living is about $1,000 per month, and the next step up (wealthy, 100 gp/month) is $10,000 per month. Technically, the average (monthly) cost of living in the United States for a single adult with no children is $2,372. So, again, we're in the right ballpark.