The question is will decreasing the cost Actually make it more useful to you?
If the mana cost was cut to 18 (so even accounting for 2 shots a second you still end up with a net mana gain per second), the DPM would close to double, but would still only be 19 compared to fireballs 15, and the damage and DPS would remain naff, and I'd argue that the latter two are far more important in most situations in the game, there just isn't great situational demand for a low damage, high ROF spell, the usual one would be swarms of enemies but with piercing spells quite so prevalent even that would not really count in its favour.
In the choice between the adjusted plasma bolt and fireball which would you choose? Fireball still does better DPS, damage per shot, and can be spammed pretty easily. The only situation where Plasma bolt would beat fireball marginally is against the overlords, but then I wouldn't use either then anyway.
Just so that we are clear on the situation, I'm not defending Plasma bolt as it is, it clearly needs changing, I just think lowering the Mana cost is the wrong way to go about it.