BILLY BAEDER has one heck of a $10 bill.

It looks like something you could get in change at the Acme, but it just might be the most valuable piece of U.S. currency printed since 1929, when bills were shrunk to their current size.

And $500,000 might be a reasonable asking price, one auctioneer estimates.

Not that Baeder, a Royersford, Montgomery County, dealer/collector and co-owner of a car-repair shop, is ready to sell.

Welcome to the wild world of currency collecting, where a "fancy serial number," a printing mistake or even just a star can mean a lot of moola, especially on a rare or historic bank note.

Find a bill with a serial number that's low (starting with 00000001), a "radar" (same backward and forward), a "solid" (every digit the same), a "ladder" (digits count up or down), a "double quad" (like 77773333), or has one of a half-dozen other patterns, and you might have a prize that blows the face value away.

Recently, Baeder paid $2,000 for a 2006 $1 bill with the serial number 00000001 that a woman got in change at Walmart.

Currency collecting is not as popular as coin collecting, "but there's a ton of people doing it," Baeder said. About 20 shows are held around the country each year, like the Whitman Coin & Collectibles Expo, held last month in Philadelphia and next month in Baltimore.

Baeder's father, William Sr., who died in May, began collecting back in the '70s, after noticing odd bills in all the cash handled at the family business, Willie's Auto & Truck Repair.

Soon, William Sr. was going to shows and getting hooked on fancy serial numbers.

As a child, Billy tagged along, building a coin collection. But his breakthrough contribution happened in the late 1990s, when he suggested they try selling bills on eBay. His father gave him a few to try, including a $100 note with a star. He hoped for $600. It sold for $1,200.

"He was floored," Billy recalls. "You gotta be kidding me," his dad said.

Billy came up with the name USA Rare at the dining room table in Hatboro, and the website was born.

Now, USA Rare does about $1 million in sales a year, with listings from a few dollars to tens of thousands.

Today, Baeder's whole collection, made up of perhaps 1,000 notes gathered over more than 30 years, is worth upward of $1.5 million, he said.

And roughly a third of that is based on just one $10 bill.

It's a 1933 $10 Silver Certificate, which bears an unusual inscription, "Payable in silver coin to bearer on demand," and has a serial number that's No. 1 and one of a kind: A00000001A.

"That No. 1 note would easily be worth about $500,000 and up," said Matthew Quinn, assistant director of currency for auction house Stack's Bowers. Any 1933 Silver Certificate is valuable, because after the government printed 552,000 of them, it released only about one-third, then soon set about destroying the rest and trying to remove as many as possible from circulation, according to Paper Money magazine. The Treasury was pushing a redesign after legislation authorized notes redeemable for any form of silver, not just "silver coin."

By June 1935, only about 15,000 remained, ensuring they'd all be rarities.

Contact staff writer Peter Mucha

at 215-854-4342 or pmucha@phillynews.com.