sammler.com, a major German collectibles site, quotes a value of €4 for a coin in average circulated condition, regardless of mint mark. At current exchange rates that is roughly US$5.
In slightly worn condition the value rises to €8.50; uncirculated is about double that.
The 1951 2 DM piece was the only coin of that denomination minted in the years immediately following the founding of West Germany in 1948. The coins were removed from circulation in 1958 so they're moderatedly valuable today. sammler.com, a major German collectibles site, lists the following approximate retail values as of 06/2014:
"D" mint mark (München [Munich]):
Slightly worn - US$33.80
Almost no wear - US$60.80
Uncirculated - US$87.80
"F" (Stuttgart):
Slightly worn - $33.80
Almost no wear - $60.80
Uncirculated - $87.80
"G" (Karlsruhe):
Slightly worn - $115.00
Almost no wear - $149.00
Uncirculated - $203.00
"J" (Hamburg):
Slightly worn - $40.50
Almost no wear - $60.80
Uncirculated - $101.00
Usually between $6 and $15 depending on condition and mint mark
Mark Saber - 1951 The Case of the Toast to Murder 1-5 was released on: USA: 2 November 1951
5DM was the only German monetary unit that was issued as both a note and a coin. Your question does not say which you are asking about. It was standard circulation, so unless it is in "brilliant uncirculated" condition or has a flaw it will have little intrinsic value.
5
Mark Saber - 1951 The Case of the Wrestler's Corpse 2-5 was released on: USA: 3 November 1952
Mark Harelik was born on June 5, 1951, in Hamilton, Texas, USA.
It is worth nothing. The Germans no long use the mark, but are on the Euro. When it was used a 50 pfennig was about 5 cents. One pfennig was a penny.
5 mark
Mark Saber - 1951 The Case of the Fashions of Death 2-13 was released on: USA: 5 January 1953
There was no 1955 British Crown (Five Shillings) minted. You possibly refer to the 1953 "Coronation" Crown.
Not much, 5 to 25 cents in circulated condition
Please check your bill again and post a new, separate question. No US bills are dated 1951.