How Ryan worked it out:

"In FB (Fantastic Beasts) it is stated that 250 million dollars was equivalent to 34,000,872 Galleons, 14 Sickles and 7 Knuts. Thanks to JKR’s incredible fondness for odd numbers, I needed to work with numbers that would often go a couple dozen decimal places. For our purposes here I decided to round off at the seventh decimal place.

A Sickle is roughly %5.88235 of one Galleon. Kind of how our nickel is %5 of a dollar. A Knut is roughly %0.20284 of one Galleon. Knowing these two numbers I was able to find the exact galleons decimal number for the strangely precise number mentioned above. That number is 34,000,872.8377278. This finally gave me a number I could divide into $250 million dollars.

To keep things simple I decided to maintain a fixed rate to the U.S. Dollar. I doubt that JKR will reveal the inflation rate in Harry’s world. Though I suspect that in books five through seven the economy can’t be all that wonderful (with Voldemort having risen)."

Exchange Rates:
( Note: These exhange rates are to the dollar, in order to find out the exchange rate with other currencies, simply compare it to the dollar)

---- ROUNDED TO 3 DECIMAL PLACES ---

1 Galleon = $7.36 (U.S. Dollar)
1 U.S. Dollar = 0.136 Galleons

1 Sickle = $0.43 (U.S. Dollar)
1 U.S. Dollar = 2.312 Sickles

1 Knut = $0.015 (U.S. Dollar)
1 U.S. Dollar = 67.048 Knuts

Muggle to Wizard:

Item	Muggle money ($)	Wizard Money
Can of soda	$0.50	1 sickle, 5 knuts
Book 1 paperback	$6.99	16 sickles, 5 knuts
A 12¼-ounce bag of chips	$3.19	7 sickles, 11 knuts
McDonalds Value Meal	$3.49	8 sickles, 2 knuts
Monthly Internet service	$19.95	2 galleons, 12 sickles, 4 knuts
DELL Dimension 8200	$999.99	135 galleons, 16 sickles, 28 knuts
Gallon of gas (/petrol)	$1.47	3 sickles, 12 knuts
Book 4 Hardcover	$25.95	3 galleons, 9 sickles
1lb Whole Coffee Beans	$7.50	1 galleons, 10 knuts

Wizard to Muggle:

Item	Wizard Money	Muggle money ($)
Omnioculars	10 galleons	$73.53
Fred & George's fake wands (each)	5-galleons	$36.76
1 Silver unicorn horn	21-galleons	$154.41
1 scoop of beetle eyes	5-knuts	$0.07
1 Daily Prophet newspaper	5-knuts	$0.07
1 ounce dragon liver	17-sickles	$7.36
Fred & George's Savings at World Cup	37 galleons, 15 sickles, 3 knuts	$278.59
Harry’s TriWizard winnings	1000-galleons	$7,352.94 !!!

Wow, so wizard money is alot stronger than muggle money! No wonder Ron was angry with Harry for buying 3 pairs of Omnioculars. It would have cost him the equivalent of $220.59!