Depends on the server... There is no set price. I sold a L25 guild with all the bank slots for 25k. Probably a bit cheap but I couldn't be bothered with it.
i wouldnt pay for a guild unless it has an active cash flow, or if it has atleast bountiful bags.
id say for a 25guild, 2k per level id pay if i needed one.
Fortunately, i dont ^^
They are good buys considering the amount of effort it takes to level, especially if they have the fish feast and caulrodn achievements.
For 25, then id think 50 was cheap and 100k more likely. All servers are different though.