Since I'm likely to play in this game and I'd like to be considered a god by mere peasants, I believe I'd like to write in an answer of 1 sp. Though, I'd still like us to start at 100 gp. That way I can own everyone I come across.
My more serious answer is that 1 sp per day seems about right to me as an expected wage. I assume that most peasants who are farmers don't actually see "pay" but instead are able to feed their family and still make about the equivalent of 1 sp per day selling the extra.
I think city based people might be slightly higher and 2 sp a day might be a decent urban wage. Given that, a gp is about a week's wage, or about 500 dollars as others have said.
Though, I don't think it breaks the economy of D&D too badly if you keep the prices in the rules the way they are and instead assume that people are somewhat more wealthy than that. If you assume that people make about 1 gp a day and that 1 gp is roughly equivalent to 50 dollars instead, you create a world where the average person can afford to hang around in the tavern buying drinks, which are often listed as 5 cp to 1 sp(or $2.50 to $5 a drink with this assumption).
This still assumes that something like a handaxe is a weeks worth of wages. Which I think works pretty well.