Not buy.
Earn. I don't know how, maybe a guild quest or the guild being a certain amount of hours old.
As for it bogging down servers, it would actually help because currently guilds make sub guilds and guild family's to get past it, and that leads to having multiple separate guilds when it could all be in one guild.
But not bought. You shouldn't have to pay to get it, you should earn the increments as time goes on, to sort of reward keeping one guild instead of each player owning their own guild (which is a waste).
This has been asked for a lot, and would probably have been added by now of it was improved. Would require work and fixing all guild related things for over 25 players.