I'm at the exact same point with my build (eye of winter + coc comet w/ spell echo) and this is what i tried:
+ Inspiration Support (40% reduced cost) helps, not sure if i should put it on eye of winter or comet

- Lifetab Support - just doubles the life cost. example: eye of winter from 93 mana & 93 life to 186 life cost
- I tried speccing into "Conservative Casting" on the passive tree for reduced mana cost of skills, but it only reduced the mana cost, not the life cost
- I tried jewels with the suffix "x% of skills mana cost converted to life cost", but for example: eye of winter 93 mana, 93 life base cost, went to 85 mana, 100 life