I never had any luck buying a guild. The last level 25 guild I saw for sale was asking 400k, which was just far and away too much cash for only 7 tabs.
On a medium population server, I can definitely say that it's much more difficult to gather the people necessary to create a guild and level it from scratch, so they generally are more expensive. Also, Blizzard has gone so far down the path of making this game all about individual-oriented play styles rather than group-oriented, you rarely see several smaller guilds merging to form more productive larger guilds because of the lack of trust and immediate benefits.
Personally, I find that it's easier to have a second account (RAF makes this awesome) that you can put all your banker mules on. If you level several characters via RAF on the second account, you should be very close to having a level 5 guild which I think will still allow you to get tab vouchers for several other bank guilds you create.
Edit to add:
I find there's no immediate value to having a level 25 guild. Yes, you can sell it back off to recoup your money and possibly make a profit if you get it cheap enough, but in my view it's more economical to simply eat the loss of starting a guild and buying all the tabs (what is that, 10k gold per guild?). You eat the entire loss for the tabs, but unless you can buy a level 25 guild for less than 30k, I would rather eat a small one time loss rather than invest 5-10x that much for a long time and no guarantee of a return.
I can take 50k starting capital buy the tabs for 10k, then double the remaining 40k in a month OR I can buy the level 25 guild for 50k and still be out 50k at the end of the month and only have an additional bank tab to show for it. Time is Money!
Last edited by Silound; December 26th, 2012 at 10:29 AM.