An icon is worth 120k. 10 currants is worth 1 favor, therefore each emblem is worth 10k. 11 emblems plus the increasing cost of 1000 favor per emblem adds up to 120k. A full set is then worth 1,320,000.
EDIT: This is the price without EHSP or the help of friends.
What's the pre-occupation with how an Icon should be worth? Value is subjective, especially in a game like Glitch. Reducing things to "it's supposed to be worth xx currants" renders player perception of value moot.
Obviously for MJK the value of 70k and 750k for his icons meets his expectations of value, otherwise he wouldn't be offering to sell them at that price.
If you are just looking for one icon and not a whole set the value of your first icon can be considerably lower if you trade all your other 1000current emblems to get the 11 emblems for that icon.
@Paxton -
With EHSP (costing 2k/5 uses=400c a use) but dividing cost by 3, it would cost assuming you bought everything from a store 3,333+400 (3,733 per emblem).
If you used alternate accounts or friends, you could do each one for this price, if you didn't, you would add on the +100 for each consecutive emblem. Either way it comes out closer to 33k~50k per emblem, nowhere near 120k.