not sure if its been suggested, or anything. and im not really sure how one would currently buy energy (iv never done it). but i think a good idea might be to make cards one could by at say walmart, wallgreens, eckards(p), right aid, etc.
similar to the Gaia cards for gaiaonline, or Maplepoints card for maplestory. say...a $10 cards get you 100 (cristal) energy. 15 gets you 150. 20 gets you 200, etc.
not sure if these prices/energy values are a little skewed (again, never bought energy), but its just an idea to work with.
also, aside from making crystal energy more accesible to people with cards, the cards could also advertise Spiral Knights a little. someone goes into walmart to buy a gaia cash card, and see's a card with these cool little knights on them. drawn in by the pic gets curious, and looks up the site later at home.
OOO accepts the Ultimate Game Card, which is available at CVS, Walmart, 7-11, and lots of others.
Also, the CE amounts you list per price are very low. You'll be pleasantly surprised when you do buy CE to find that you get 1600 for $5, 3500 for $10, 7500 for $20, and 20,000 for $50.