New Team Mana Sustain Item

I was thinking about the old Mana Manipulator and had an idea for a mana sustain item.

Mana Manipulator (in lieu of an idea for a name):

Recipe: Forbidden Idol + Catalyst the Protector + 670g (2600g total)

+300 Health +350 Maximum Mana +50% Base Mana Regen +15% cooldown reduction

UNIQUE PASSIVE– VALOR'S REWARD: On leveling up, restores 150 health and 200 mana over 8 seconds.

**UNIQUE PASSIVE - MANA MANIPULATOR: ** Forms a tether with the nearest allied champion within 500 units. When a tethered champion casts a spell, 15% of that spell's mana cost is consumed from your mana pool instead. UNIQUE ACTIVE - MANA TRANSFUSION: Consumes 20% of your maximum mana pool (or all, if your current mana level is below 20% of your maximum mana pool) to grant half that amount to the tethered champion.

Analysis: 300 Health @ 2.67g/1 Health = 801g (200g for 75 Health excluding items built into Mana Manipulator) 350 Mana @ 1.4g/1 Mana = 490g (20g for 50 Mana excluding items built into Mana Manipulator) 50% BMR @ 5g/1% BMR = 250g (100g for 25% BMR excluding items built into Mana Manipulator) 10% CDR @ 30g/1% CDR = 300g (0g for 5% CDR excluding items built into Mana Manipulator) That would make the Mana Manipulator passive worth 350g.

Reasoning: I think it would help out with champions that have higher mana costs, and it would allow casters in bot lane to do their thing if this item was picked up early enough. It costs about average for a legendary item, but it has a good build path for most supports, providing health and mana sustain so that you don't feel like you haven't bought anything until you actually get the item you're building towards. The passive would also automatically target the champion nearest to you, so it would encourage good positioning to properly utilize this item. The active is there in case your teammate needs to cast a high-cost spell and they're sitting on practically no mana.

All hypothetical here, just bouncing some ideas around. What do you all think?