highsorcerer wrote:Actually, I don't think James was a professional Quidditch player; I simply stated that it was one career path that allowed a person to earn a lot of money very quickly.
my bad.

Entertainers (including pro athletes, singers, actors, and writers) who become breakout stars can make lots of money quickly (though it is hard to break into and you need some talent). Other ways include developing lucrative patents, high-gain / high-risk investments, and crime.
On the whole, I have to think the Potter money was obtained the old-fashioned way - it was inherited.
On the whole, I agree with you (I think there's a good chance there's an inheritance), I don't think we should rule out that James and Lily just had good old-fashioned jobs for about seven years.
Imagine with me for a moment:
James and Lily live in a house that's been in James's old wizarding family, and therefore have no mortgage.
The Potters, like so many other wizarding families, don't have electricity, therefore cutting out electricity bills, cable bills, internet bills, etc.
The Potters, like so many other wizarding families, either don't have a car or have only one car they use only from time to time (take away car payment, car insurance, gas, and upkeep)
The Potters, like so many other wizarding families, almost always communicate using owl post, rather than phone (take away phone bill.)
So, since we've taken away almost all regular monthly bills, it's quite possible that a large portion of the money James and Lily did make could go straight into savings. If this is the case, they might not have needed extremely high-paying jobs to save a LOT of money over a period of about seven years.