In general an estimated value can be used to allocate the purchase price of intangible assets that are not specifically identified. ie. allocation between goodwill, convent not to compete, customer list etc.
The overall process of breaking down the value of an asset acquisition is outlined in IRC 1060, also Form 8594 - Asset acquisition Statement provides instructions as to the proper allocation of asset values from Class 1 assets (cash, general deposit accounts), Class 2 assets (traded personal property) to Class VII assets ( Goodwill, going concern value, Section 197 intangibles)
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Can you give me some of sources?
Major general source of instructions are Form 8594 - Asset acquisition statement under section 1060 - http://www.irs.gov/instructions/i8594/index.html