I'm feeling nerdy, so I decided to calculate the Exchange rate to make it easier for Bungie if they decide to add a Bureau de change in the tower.
A strange coin exchanges to 4270 glimmer as both Xûr and Banshee 44 sell heavy ammo synthesis for one strange coin (for five) and 950 glimmer. Providing they are honest tradesmen, one strange coin is worth 950*5 = 4270 glimmer.
Motes of light are slightly more difficult to calculate. An exotic engram for armor costs 23 motes, and a piece of armor costs 13 strange coins. I know it's unreliable because an engram is like a lottery ticket but it's the best comparison. There's also the Speaker's class items but they have a style cost added on. 13/23=0.565 (there are loads more decimal places but I'm only typing to 3 dp to make it faster), so a mote is 0.565 of the cost in glimmer for a strange coin, making 2684 glimmer.
That leaves glimmer as the Vietnamese Dong of the currencies, nearly valueless. If one strange coin equals 4270 glimmer, then the suros regime is worth 109,250 glimmer. 4270/109250=0.04, so one glimmer = 0.0435 strange coins
So to round it off=
1 strange coin = 4270glimmer
1 mote of light = 2684 glimmer
1 Mote of light = 0.565 strange coins
1 strange coin = 1.769 Motes of light
1 glimmer = 0.04 strange coins
If you think I've done something wrong, please feel free to correct me.
Edit: It has been pointed out to me that Xûr sells 5 synthesis for a strange coin, so I have edited it accordingly.