Having values or some state in a database

Suppose I have a table with several price values in it ID Int, Primary Key, not null AveragePrice Decimal, not null MinPrice Decimal, not null MaxPrice Decimal, not null Each price value can be known and present (represented as a positive value) or it can be * infinite (without upper bound) * unknown * irrelevant * customer specific There may be more even states. **How to model this in a database?** Currently these states are modelled as negative constants, e. g. a price of -10 means "unknown", -20 means "irrelevant". Since prices are normally positive this kind of works but I hope there exists a more elegant solution.