Unlike Badges of Justice which will only be legal tender for Level 70 item rewards, or Arena Points, which will be wiped clean the moment we ding 71, Honor will still be usable to purchase on-level rewards. Of course, we can expect inflation to apply -- a pair of Guardian bracers today cost 11,794 Honor and 20 Warsong Gulch Marks of Honor, but data suggests that the Deadly Gladiator bracers will cost 39,400 Honor and a 1660 personal Arena Rating. The Guardian belt and boots currently cost 17,850 Honor and 40 Eye of the Storm / Arathi Basin Marks of Honor, but looks like they'll cost 62,000 Honor and 1630 and 1720 personal Arena ratings respectively.
The current Honor cap is at 75,000, with each type of Mark of Honor capped at 100. This means that at the cost of the boots and belt, you'll only be able to purchase one piece when you hit Level 80 and play a few games through Season 5 when it starts on December 16. It's still not a bad deal if you've stored away all that Honor to begin with. And this is only if you choose to go for the best of the best... the Hateful and Savage Gladiator equivalents should be much more affordable. I wouldn't trust the current prices just yet -- anything can change once the PvP item vendor finally becomes accessible in Lake Wintergrasp. That said, any Honor you will have banked for Northrend will be a good investment.