Generally it's not hard to master or hard to shoot.
And, as I've stated, great effort for great reward is not balanced in the case of extremes. The idea is that reward should scale to the price, but this scale should be exponentially decreasing. Something that takes 100% more effort should only be 50% more rewarding, to give a loose example. Otherwise you run into a situation where everyone is making heftier investments because the reward scales to or is greater than the price. The strategies that emerge will be entirely about mastering one's ability to make that large investment to the point that the price is considerably lower and thus the price no longer matches the reward. To this end, mastering a skill must be accounted for in the balance.
Furthermore, you're ignoring the point. The 30-damage value lets us make the sniper itself less difficult and aggravating to use. Suddenly the sniper is less of an annoying specialty and instead has more of a real, fleshed out role. It isn't about making that large investment of effort to get that large reward of an instant kill, but instead about making smaller investments to get appropriate rewards.
The two kinds of damage-types that emerge for classes are the finisher and the weakener-- one that does more damage on weakened targets and one that does less damage on weakened targets. A sniper that is no longer instant kill can fit suitably into one of these roles, and thus be balanced around that, and thus be more interesting on the whole.
You may work hard to score that instant kill, and that reward may or may not (usually not, being much greater than) fit the price, but, in the scheme of things, that instant kill is detrimental to the game as a whole. It gives someone the ability to stand alone instead of forcing them to stand together (as is the only way to get the gameplay to that state where the players of a team actually do stand together). When you break that mold, it breaks for everything. You must have consistency and strength in all areas with regard to this sense of teamwork or else the balance is lost. Teamwork becomes something you must go to great lengths to organize and provide to the game instead of something that naturally arises of the game.
In short, in a team environment, the entirety of balance is weeding out solo-players by any means necessary.
