Harry "the billionaire" Potter!
I find that a common problem in HP fanfic is making Harry a bllionaire, or at least a 100 millionaire. Unfortunately, that's impossible. The Wizard economy in Britain is simply too small for any one person to be that rich in 1991/2, or even today! If you assume there are 100,000 adult wizards in Britain (most probably the number is closer to 85,000 - those under 19 make up about 15,000) and the average wizard earns the same as the average muggle, then the total Wizarding economy has a Gross Domestic Product (population multiplied by the yearly income of the population) of only 2 billion galleons. It is impossible for someone to accumulate half the money spent in an economy in a year.
As an example, the US has a GDP of 16 trillion dollars in 2014 and the richest person in the US is Bill Gates at $89 billion or .0055% of the GDP. The top 10 richest in America combined only reach .025% of the GDP. Applying those numbers to the wizards in Britain means the richest wizard (probably Sirius) has only 1.2 million galleons. And the top ten would share 4.25 million galleons.
Great Britain has a quite a few billionaires, far more than the USA, but their riches, for the most part, come from OUTSIDE Great Britain because of the generous-to-foreigners tax code – Russian oligarchs, and foreign energy and commodity magnates with incomes from outside Britain. And most are immigrants. The British GDP in 1992 was 1.58 trillion dollars with 58 million citizens, making the average yearly income ~£10,000 (check google) or ~2,000 galleons at an exchange of 5-to-1. Gerald Cavendish, the Duke of Westminster, was the the sixth richest U.K. citizen with an $11 billion fortune (£5.6 billion) or .0069% of the British economy (the five richer citizens were all immigrants with incomes from GLOBAL markets – the Duke's income is entirely based in England). The Queen of England had only £300 million, which would be 60 million galleons. In 2015, J.K Rowling hit £580 million with 90% of that coming from non-British book sales and Hollywood movie income.
You can't use the Muggle numbers to justify the wizard fortunes because then all the businessman wizards would have to know that 99% of their income came from the Muggle side of their economy, and they would value everything in pounds, not galleons (Sheer volume would drive the galleons out of the market). Only a complete idiot would deliberately and knowingly plot to wipe out the source of 99% of his income, no matter how much he despised it! (Sort of like cutting off your nose to spite your face.) For example, both Thomas Jefferson and George Washington detested slavery, yet maintained large estates employing slaves — because they would go bankrupt if they tried to run their estates without slaves. The triumph of practicality over morality!
Of course, you can make the argument that some wizards are smart enough to invest and own some Muggle companies, boosting their net worth to higher numbers, but you would still see their income staying well below the numbers you would see on the Muggle side.