It first depends on what state you are dealing with. This expression shows up in standard forms in Georgia, where it is not defined. You can read the associated statutes (Georgia Code, Title 53) especially the definitions, and it won't tell you. The probate court rules also don't tell you. So in Georgia, it would be "what a reasonable person would conclude" (good luck there). Searching for legal blogs that might give a hint, this article refers to "Significant assets that are solely titled to the decedent- property, automobiles, boats, homes", which seems obvious – they don't suggest a lower value, because that is not legally determined and they don't want to get in trouble for advising that $1,000 (or $500) is "an insignificant value". The purpose of the form is to put beneficiaries on notice as to what is at stake, and for purposes of that form, it is an estimate, not a binding promise. You have pretty much identified the indeterminacy of the law (in Georgia).