How does Material Exchange work?

I have the option of trading in some Spirit Bloom (or any of the other materials—Spinmetal, Helium Filaments, Relic Iron) with the Crucible Quartermaster (I believe some other vendor does it too), but it gives me no indication what I'd get in exchange. I imagine it's some amount of rep and perhaps marks, but what is it? And if it's variable, what are its deciding factors? Do all kinds of elements have the same conversion rate?

Note: As of the December 1st update material exchanges now work in the opposite direction, where marks are used to purchase materials.

What is the exchange rate for marks to materials?

Solution 1:

You can use 10 legendary marks to purchase 20 of any planetary material from the Vanguard Quartermaster. See below to locate the quartermaster.

Faction rep (per each of the 3 factions: Dead Orbit, Future War Cult, New Monarchy) can be gained by speaking to the respective faction rep in the tower, as seen on the below map. The exchange rates are as follows, gaining rep for the faction rep you exchange with:

5 Motes -> 100 rep

25 Weapon Parts -> 50 rep

25 Old Armor Materials✝ -> 50 rep

25 Unified Armor Materials -> 50 rep

Special Ammo Synth -> 25 rep

Heavy Ammo Synth -> 25 rep

✝ Old Armor Materials includes Plasteel Plating, Sapphire Wire, and Hadronic Essence.

EDIT: The below exchanges are no longer valid as of "The Taken King", September 2015.

You can now go to the Vanguard/Crucible Quartermaster and exchange 10 marks for:

20 Spinmetal

20 Helium Filaments

20 Spirit Bloom

20 Relic Iron

The Crucible Quartermaster is in the middle area of the tower before the class Vanguards (opposite Lord Shaxx). On the map this is in the blue area near the icon that says Crucible.

The Vanguard Quartermaster is in the area near the shipwright. On the map this is in the red area and marked Quartermaster.

EDIT: The below is no longer true as a of a patch in late 2014.

Each time you exchange 50 of each material to either the Crucible or Vanguard Quartermaster, you get:

+5 Crucible/Vanguard Marks (as long as you're below the weekly limit)

+25 Crucible/Vanguard Reputation (these change if you have a faction class armor equipped)

+250 Glimmer

Note that you are also capped at 200 total marks for each faction, and 100 per week for each faction.

Solution 2:

These answers are all from before the end-of-year patch. Now, materials exchange only works the other way: you can buy materials, but you can't sell them.