UPDATE: the newest PTR has them exchanging for 185 honor. Sad day.
Currently, on the PTR, each mark will be worth 2k honor. Yes, that's a lot. Let's compare.
Right now, a PVP Mount costs 30 AV marks, 30 WSG marks and 30 AB marks. After, a PVP Mount will cost 50k Honor, which is basically 25 marks. (UPDATE: nope ... not any more)
So ... yeah, save those marks if you can. More prices, as examples:
Lvl 60 Insignia of the Horde - 2.8k (less than 2 Marks, but you can also get it from a quest in Hellfire, and is replaced by Medallions of the Horde)
Monk's opinion: Because you can only wear some of these items at 70 (rather than 68), you might only be wearing them for a few lvls, depending on if you run dungeons or do group quests. Might want to save the honor for lvl 80 items if you're not using the points on a mount.
-- Edited by Monk on Wednesday 3rd of March 2010 02:38:31 PM