You can calculate that guys:
Potential added value of lust in bonus damage done per mana point spent.
Potential added value of heal in hit point healed per mana point spent.
I already did the maths for you in a previous post you can dig that up if you are lazy or do them yourself but long story short:
Even if heal costs 1 mana only, lust still has a lot more value per mana spent than heal. The only way you can change that is by also increasing lust's mana cost to make it more in the line of other spells in the game (which it isn't).
Now if you really want to fix the balance I recommend making the smallest changes possible to every single spells in the game to achieve it and not changing the mechanics if you want to preserve the feel of the game.
Basically you want the game to feel the same as much as possible radical changes will just not work for me...
Also I may add that balance in my opinion is not necessary at this point, playing humans is a handy way to give ourselves a small handicap against weaker foes and/or force practice all the strategies that do not involve the overlly famous T3 powerspike.
If you balance the game you are effectively removing that one click option and we have not real handicap mechanic to replace that (like the handicaps in starcraft or warcraft3) war2 sadly does not have these options.
And no, mages are just in no way better than dks if you just take the time to do the maths:
Spoiler
Coil = AOE spell that does 1 hit point value for 1 mana spent. (100 mana, 50 dmg 50 heal) it is by far the best and easiest one shot spell to use in the game and is free and already researched. Blizzard has less spreading than d&d and even if it does more damage when the shards land directly on target units (about 10% more after empirrical testing) the splash is very very low making it a very unreliable spell against small amount of targets since you have to be more lucky to have that shard right on target. If you test the two spells on isolated units and count all the times the shard miss the target too you will see it is actually better to have d&d in these situations.
So not only d&d is more reliable but it also does more than 41% more damage to farms which is just crazy (empirically tested as well feel free to test all I am saying for yourself).
Haste is a much better spell than both slow and invisibility it is probably the second best spell in the game after d&d and it is cheap and fast to research and is also cheap in mana.
There is no denying imbalances exist at this point but at the same time no one that really loves the game really cares about them, there is something fun in playing the underdogs and we have no other easy handicap mechanics built in the game anyways.
Just like the most imperfect and imbalanced version of GOW is still the most popular and played version, you can bet that top tier players all will always prefer playing the classic imbalanced version of war2 if the choice is ever given to them especially if the balancing changes the way the game feels.
And maybe that is not such a bad thing