Non-transferable doesn’t mean that there isn’t a market.
The citizen-share comes with the citizenship-shareholdership; thus, the market price floats at the point at which the Empire can fulfil all the obligations it has towards the new citizen-shareholder purchasing that share, which is to say, at the point at which the return on the capital paid for the citizen-share plus the estimated future income stream from (the average) citizen-shareholder ESF payments neatly balances the per-capita cost of providing for those obligations [i.e., the Citizen’s Dividend plus all governance functions], plus reserves.
(The demand side of this particular equation is dependent upon how many qualified would-be citizen-shareholders there are at any given time.)
The market-clearing price, therefore, is that at which the Empire acquires the largest number of new citizen-shareholders to which it can meet its obligations, and everyone’s a winner.