Ok, the Biker DLC is up.
The clubhouses start at $200,000 and go up to $495,000. There are two types (single or double storey). They're spread around the map: plenty in LS, but also around Sandy Shores and up in Paleto Bay. You can spend a fortune on them though because you can customise murals, interiors, etc. You can put a bike modshop in as one of the options. You can take them up close to $1,700,000 I think.
I haven't seen the MC missions yet, but being president gives you access to Clubhouse Contracts and the Open Road website. I'm assuming the former is like VIP missions, and the latter like the SecuroServ crate missions.
The new clothes are pretty much what you'd expect (jackets, vests, etc, although there are some sporty options and also puffer jackets).
There are new weapons:
Pipe Wrench ($6363)
Mini SMG ($7921) - Skorpion
Sweeper Shotgun ($13,261)
Battle Axe ($8455)
Compact Grenade Launcher ($40,050)
Pool Cue ($5562)
Of the weapons, I think the pool cue is a regular melee weapon, while the Battle Axe and Pipe Wrench are the ones you can use while riding.
For bikes there are a heap. Mostly choppers, but there are also two mopeds, an ATV, a trike, the "Manchez", and two sports bikes (a street fighter and a drag bike).