Based on what I can find this is the most (and only) Harley Davidson Motorcycle in the top 10 list.
When a “barn-find” Harley-Davidson racing machine and sidecar is found after 50-plus years in storage in Australia and then sent to auction, the bidding is sure to be fierce. The expectations were that the bike would go for about US $100,000.
But the bidding was greater than expected. The bike set a new Australian auction record with a final winning bid of AUD$600,000 (US$420,000).
What caught everyone’s wallet on fire? A circa-1927 FHA 8-Valve V-Twin racer complete with its scramble-type sidecar. (The motorbike’s side car was added to the motorbike after it made to Australia and can be angled to adapt to what tracks the bike was running on according to the auctioneer.) 
The 8-Valve racing Harley is one of the rarest and most collectible models in HD circles.There were fewer than 50 built which makes it an exceptionally rare machine anywhere in the world. So for one to turn up in an Australian shed after decades certainly set the motorcycle collector world abuzz.
These powerful bikes were only offered to up-and-coming racers of the times and were not available to the general public. “To many collectors the engine is really what makes this bike. This was probably the holy grail for Harley-Davidson in this era” stated Mr. Boribon the auctioneer.
As noted above the fact that less than 50 were built makes it rare, the fact that a lot of race bikes ended there racing lives in spectacular fashion contributes to even lower numbers left in existence. I am sure more than a few of the 50 blew up or crashed on the track. So based on those facts it is very surprising that very few 8-Valve racing bikes have survived from that era.
This is the only Harley in the top ten most expensive bikes at auction coming in at
1 – US$580,000 – 1910 Winchester
2 – US$551,200 – 1915 Cyclone Board Track Racer
3 – US$480,000 – 1939 BMW RS255 Kompressor
4 – US$463,847 – 1922 Brough Superior SS80
5 – US$452,234 – 1926 Brough Superior SS100
6 – US$450,000 – 1958 Ariel 650 Cyclone
7 – US$448,156 – 1929 Brough Superior SS100
8 – US425,943 – 1939 Brough Superior SS100
9 – US$420,000 – 1927 Harley Davidson FHA 8-Valve V-Twin racer
10 – US$394,101 – 1934 Brough Superior SS100
(Bumped from the top 10 – US$383,317 – 1949 Vincent Black Lightning)
I JUST WANT 2 RIDE!! 