There's no item per se that increases unarmed damage by one size category.
In MoF, there is a magic item called a Monk's Tattoo that does not take an item slot that allows you to treat your effective Monk level as 4 higher than normal for purposes of unarmed damage, fast movement, and Monk AC bonus. This effectively increases your damage up to the next die size (d6 > d8 > d10 > d12 > d20 for Medium sized characters), adds 10 ft to your base speed (20 ft if you're lucky) and +1 AC.
This isn't the same as a size category increase, but it's close (actually it ends up being a little better in terms of damage than a size increase). This is probably the closest you can get with existing items.
Mastery of one of the martial arts styles in OA gives increases your damage by one size category according to the DMG tables.
You can probably start with the Monk's Tattoo as a reference point and design something yourself. IIRC, The Tattoo went for about 80,000 gp if I remember, which I think is a lot for +10 ft speed, +1 AC, and +1 die type for unarmed damage. The d12 > d20 increase is pretty big (avg 6.5 -> avg 10.5, difference of 4 pts), but the others are not very significant.
A +1 size category increase for unarmed damage item is difficult to price since there are no good items to compare with. The extra damage is not very big at all until you get up to about the 2d8 range. Even a 1d8 (avg 4.5) to 2d6 (avg 7) is only about +2.5 points of extra damage. Definitely nice, but hardly worth writing home about. But 2d8 (avg 9) -> 4d6 (avg 14) adds about +5 damage average which gets doubled on a crit. Very nice, but still, probably not worth a huge sum of money.
This is hard to price since the benefit you get is more the larger your current unarmed damage die is. I do agree with the previous estimates -- probably about 20,000 to 30,000 is fine.
We need more monk items, kind of like what they did in Neverwinter Nights....