Halo: Reach has this interesting little "armor store" to help you customize "your" Spartan's appearance. This is an increasingly common thing to have in games, and builds from Halo 3's multiplayer armor customization (particularly given that you play "your" Spartan in the Campaign this time). To gate off everyone from having the best looking armor items, items in the store are locked by various prerequisites. The two most common prerequisites are rank and credits (the store's "currency"), which are bound together by the game mechanics.
In Reach, credits serve as the individual XP or skill points in a very poor MMO grind. Ranks, the equivalent to most MMO's levels without any of the RPG benefits associated with levels, are unlocked by cumulative lifetime credits earned.
Because of how credits and rank are bound together mechanically, there are a few, mostly cosmetic, items in the store with prerequisites that "defy" those bounds, probably for psychological reasons. The example that I'm going to use is the "Birthday Party" armor item. This item makes it so that confetti explodes from your corpse when you die in multiplayer games. I'm a mediocre to average player, plus I have a hard time paying attention to my health level when I'm amidst combat, and so I tend to die a lot. "Birthday Party" on my own armor would certainly improve my own enjoyment when playing Slayer battles on Reach (and yes, I tend to play the Halo campaigns with the "Grunt Birthday Party Skull" to lighten the tension there as well). So "Birthday Party" unlocks at Sergeant, a rather low rank, but costs, if I recall correctly, a half million credits.
Even though "Birthday Party" unlocks at Sergeant, there is no chance that a Sergeant would have half a million credits. If you examine the Halo: Reach Rank Chart Sergeant unlocks at 20,000 cumulative credits and even if you saved all of those credits you are still well short of the necessary half-million. The first rank where you would actually have a potential opportunity at having a half-million credits banked is Lt. Colonel Grade 2, with a requirement of 510,000 cumulative credits. There's nearly twenty ranks (and seemingly a million years of grinding from my perspective) between the two ranks. Psychologically it does look nicer that the item is available at Sergeant, if you just had enough credits, but from a game mechanic standpoint the game might as well have put a Lt. Colonel Grade 2 requirement on that half-million credit item.
What's also curious is that because Ranks are bound to cumulative credits there are fewer opportunities for Bungie to award armor items, like "Birthday Party" at the actual minimum rank the store suggests should be possible. Certainly there could be a "fire sale on birthday parties", perhaps on people's actual birthdays, that could drop the price to a much more affordable couple hundred credits or so, but Bungie probably is much less likely to actually give players large sums of credits. The resulting "field promotion" from a large handout of credits would probably anger some of the more hardcore Halo grinders.