I stopped playing Nami because I thought her mana costs were too high. Her base mana pool and abilities has her going oom in really short order, especially on her heal.

I agree that her Heal is a bit heavy. If that was fixed, no problemo. Other than that a Chal should fix it.

after sightstone, athene's is the first thing i build to compensate for 1. her long cd's and 2. mana issues. works well but sucks if your adc is not getting kills (i.e. no assist). espeically now with the added cost of the ruby into sightstone.

Morellonomicon is also another option i go if I feel I don't need the ap resist. solves the same issues.

still enjoy playing nami but becoming more of a chore.

The only really expensive ability is her heal, but since it's both a heal and solid poke (potentially for all four in the lane) it's actually still pretty darn efficient. Sure, other supports might be able to cast more spells, but will they have the same efficiency? Usually not.

Well, I suppose you could argue her ult is expensive at higher ranks, but usually that's not an issue by that point.

I never have any mana problems.
Are you using the heal without bounces on your carry? It's not efficient that way.