List of the most expesive coins by county-

Bulgaria- I've seen the 1912 100 gold levs sell for up to $4000 on ebay.
USA- 1913 Liberty Head Nickel $5,000,000
...

Actually USA 1933 $20 Double Eagle at $7.6M.

As for the British record, I think this was the one - sold at Spink, in 2006 - the Edward III Double Florin.

British record sold at Spink auction house

£460, 000

This coin was found by a metal detectorist!

The last sale of a proof 1930 penny [Australian] went , from memory for $1.2 million. [AUS]. There are only 6 produced of these, I think only 2 or 3 are in private collections.

yeah! I remember drooling when I heard about that news!

The Canadian 220 lb gold coin is worth a good amount. $1 million face value and about $3.4 million in gold value

German coins are bargains, it seems. The most expensive piece from the Federal Republic of Germany is the 5 DM 1958J; expect to pay about €4,000 for an unc piece.

If you include former countries in this area, prices are a little higher. There is the 3 RM coin, for example, that Saxony issued in 1917 to commemorate the 400th anniversary of the Reformation; that costs about €70,000 to 90,000. At a Künker auction about €190,000 were paid for a 10 ducat "Portugalöser" (Brandenburg 1584). The most expensive piece that I know of is the Prussian ducat from 1803 (gold 3.22g) that was sold, at a Peus auction, for about €200,000. http://ars-antique.com/img/galleries/33/01.jpg

These days Russian coins, inclusing the USSR, seem to be "hot". Last month, Künker just sold a 20-medal set from the early 19c (featuring Tsar Alexander I and various scenes from the war against Napoleon) for €380,000. Also in March, Gorny+Mosch sold 59 Soviet pattern coins, separately, which all in all brought €1.3 million. Among the patterns was a 5 kopek piece dated 1947, starting price €25, which was ultimately sold at €35,000 ...

Christian

When I should be doing something more constructive I waste time by looking through my old Gadoury's catalogue on Royal French coins, looking at all the great stuff I'll never afford. (1987 prices)

Some biggies:
1715 H écu aux trois couronnes, 280,000 FR in SUP (divide by 5 for $)
1691 Pau Louis d'or à l'écu du Béarn, 180,000 FR
1648 D Double Louis d'or à la mèche courte, 150,000 FR

Then of course there are all those that are simply marked HORS COTE, too rare to put a price on:
1715 A + 1715 & 1/4 écu aux 3 couronnes
1718 BB 1/2 écu de Strasbourg
1715 9 écu aux 3 couronnes

Some of these coins are only referred to in 19th century catalogues and haven't been seen since.....