Guild stones range from about 2-5m depending on shard, but they can often exceed that range depending on the desirability of the name, and any personal relationship/attachment an individual has to the name.
*edit*
Forgot to mention that guildstones relating to historical events, events, or are related to roleplaying will also generally be valued higher. So for instance "9 of Elders and Nystul's Knights" may be worth more to someone b/c of its reference to Nystul a figure of historical significance.