while the pc on the guildstone would normally be roughly accurate you must take into account the RP factor. Guild Names which either still exist or in this case have an RP ring to them are often quite desirable for that particular community, and it isnt unusual to see them sell for upwards of 20-50m depending on the individual or group interested in them.
Generally those in the higher end of the price range i mentioned above are ones including names of historical reference as they are even more desired by the rp community for the ease of blending them in with game history, etc.
its a nice guildstone though and i would try to sell it for at least 20m-30m. in the event you need the gold quickly or cant find a buyer you could always reduce the price.