Any collector value would depend on the country of origin of the coin, the denomination of the coin, the condition of the coin and how far off the mark the problem is.
A "Mule" coin is where the obverse and reverse dies are not matched as per the official specifications for the coin.
More frequently, it is, for example, a ten cent coin struck on a five cent blank or planchet.
It is a mule.
If it is a proper one and not a mule it is worth £614
A mule nickel is a valuable error coin worth several thousand dollars. These coins were created when a buffalo nickel die was paired with a Jefferson nickel die, resulting in a mule coin with features from both designs. The specific value can vary based on its condition and rarity.
by the looks of things the answer to your question is yes it would be a mule. A "Mule" is a coin whose obverse is not matched with the official reverse.
If by a "mule" coin you mean for example, a 1999 Australian 2 Dollar coin reverse struck with the obverse of a different coin, like a similar size New Zealand coin or, on an Australian 10 cent blank, this is an accident of the minting process and a valuation cannot be anticipated. Examples of your coin have sold on eBay for up to $26 AUD. A reputable coin dealer will be able to give a more accurate valuation based on inspection of your coin.
More information is needed, please be more specific and post new question.
This "mule" coin came about when the obverse of the Indian Quarter Anna coin and the reverse of the Australian Halfpenny were used together. There is thought to be 10 or less of these coins in existence. Both the Indian coin and the Australian coin were minted at the Calcutta Mint.
A Sacagawea coin with no date is likely a minting error known as a "mule coin." These coins are valuable to collectors and can potentially be worth more than its face value, depending on its condition and rarity. It is recommended to have the coin authenticated by a professional to determine its value accurately.
All error coins need to be seen. A MULE coin is a coin struck from two different dies not intended to be used together such as a dime obverse with a cent reverse. I don't recognize the term"Bubble-Head" but the planchets used for Eisenhower dollars are known to have "gas-bubbles" in some of them. Take it to a dealer of an accurate assessment.
The value of a silver coin is always changing because the value of silver is always changing. See the related link below for a silver coin value calculator. This does not give the actual value of the coin but it does give the value of the metal used to make the coin. This is know as the melt value.
What you probably have is a novelty coin made by machining two coins to fit together so to appear to be just one. They have some value but it is generally limited to whatever you are willing to pay for them. However, new things are always turning up in the coin collecting world so I would suggest to take the coin to a coin shop and have them verify what it is.
the smallest value of a coin is a penny.