Suppose someone steals a part of a sellable object (e.g. stealing a wheel from a car dealer) or one item in a bundle (e.g. taking half of a bag of donuts)? Or suppose the value of the item cannot be easily measured, such as if someone were to steal the Declaration of Independence? How would the value of the item be determined for purposes of sentencing and classification of crime?
The value of items stolen is determined as a factual matter at trial based upon the evidence presented at trial (unless the prosecution and defense stipulate to a value). Usually, the exact value isn't material, only which statutory range of values of several possible choices, the good in question belongs in according to the trier of fact.
This could include evidence of the offered retail price, the purchase price, the testimony of the owner, market quotations for goods in which there is a ready market, and expert testimony of property appraisers.
Usually, the jury would make this determination with either a special verdict on the question, or by choosing a particular charge to convict upon, when there are several lesser included charges brought based upon the value of the item stolen.
Sometimes, the matter is left to the judge, for example, in the case of a federal judge applying the sentencing guidelines in a case where there is merely a conviction of a theft offense without a specified value.
A determination that impacts the maximum possible statutorily authorized punishment must be made beyond a reasonable doubt and must be made by a jury if there is a jury trial.
A determination made for sentencing purposes by a judge within the range of punishments authorize by the crime of conviction is often made in a post-trial hearing before a judge on a preponderance of the evidence basis with looser evidentiary standards than apply at trial.
A trial court trier of fact's determination of the question is upheld on appeal if there was any evidence in the record consistent with, or which could support, the determination made at trial.