I agree that there would be very little seignorage in any market-based system to create money. An important question is, though, what concrete mechanism it is that drives up the cost of creating money in a given market-based money creation system. I gather that in the Bitcoin system, it is the fact that people spend huge amounts on hardware that hosts the Bitcoin software. This is, if I understand correctly, wasteful to some extent.

Another market-based system which wouldn't be wasteful in that sense is one along the following lines. Call the new currency X-coin. Say that new X-coins are created now and then (either by a central authority or according to a pre-set process). Every time a new X-coin is created, people may bid for it in dollars. Now since each time a new X-coin is produced, the value of all other X-coins is somewhat diluted. Therefore, the dollars that the purchasers of new X-coins spent get distributed to existing X-coin owners (in proportion to the number of X-coins they own).

That would be a market-based system that wouldn't be wasteful. Note that it depends on the existence of another currency, dollars, and wouldn't work if the X-coin system got world monopoly (or perhaps not even if it became relatively large).