Buying a Weapon Proficiency, and 3d6 dmg weapon

[Chun-tzu]

Yes there are weapons like that. A Heavy [FR] Merc. 2 Hander[S&F] does 4d6 damage. Of course you have to use 2 non-core books.

The stats on the Mercurial Greatsword have been changed in the Sword and Fist errata to 2d6. It differs from a regular Greatsword only on a critical.

Thus, a heavy Mercurial Greatsword (if your GM allowed such a thing) would do 2d8.

 

[Bastoche]

The sun blade more or less confer the exotic weapon feat (bastard sword) IF you already have the proficiency in short sword.

What your DM accepted is tremedously ridiculous. The weapon is ridiculous (even more than mercurial weapons IMO) and giving feats is even more ridiculous. No matter the amount of cash in the trade. IMO, feats shouldn't buy "buyable" by any means. Except if the opportunity is oppenned to all. For example, in a campaign we're starting out soon, the DM gave each player a free exotic weapon proficiency. There's three characters, they took fullblade, shuriken and blade boots IIRC. (I'm the one who took shuriken)