How much is a law firm's trade mark practice worth?

A friend of this blogger, who wishes for professional reasons to remain anonymous, raises an unusual issue here:
"Reading bits and pieces on the exercise of trade mark valuation for various purposes, ranging from structuring balance sheets, IP due dilligence, company takeovers, intangible assets assignments to obtaining funding, bank loans or securing debts, I was thinking about how such an exercise could differ (if it in fact does) when the subject-matter of the transaction is trade mark portfolio management -- when a law firm 'buys' another law firm's trade mark 'bank'.

I wonder whether there exists some form of literature, guidance or thoughts on the matter which have somehow managed to escape the dark rooms of intra-lawyer transactions. I am pretty sure this is not a transaction which has never seen the light of day.

I would think that the factors involved in identifying the value of such trade mark responsibility take-over would be
* the depth and diversity (in terms of clients, classes and/or geographic region) of the portfolio;  
* the TM owners' profiles (are they large, are their operations international?); 
 * the TM owners' diversity of business; 
* the nature of the TM owners' businesses (would they require frequent filings or licence recordals?);  
* renewal maturity;  
* the possibility that some of the TM owners will not stick with the new law firm;  
* cross-selling possibilities (will the move benefit other departments of the law firm, which will get the chance to approach the client/TM owner?).
Is there a method of translating this into numbers? Or would one just say 'listen -- our TM prosecution practice has brought in X in the past 3, 5, 10 years and, based on this, its revenue projection for the next years is Z? And is that a "just" or are there specific rules for such projection too?".
Can any readers offer advice, or make some pertinent comments?
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