I have no simple answer to this question -- it could very much depend upon the costs of triggerable powers and the costs of cards I like to play together. If I have a bunch of ominous eggs, I would want to avoid cost 0 and cost 1 minions (though I still might go for cost 0 and 1 in non-minion cards). In creating a magic immune deck, I place higher value on cost 3 minions (that play on the same turn as the cost 2 astral armor that grants them immunity) than I would otherwise give them.
Given the three point first turn, I think a good deck almost has to include a significant number of cards that can be played with three (or fewer) power points. On the other hand, in some ways, 3 power is the worst cost for a unit -- it leaves 2 power points to be potentially wasted, it cannot play with another cost three card, and tapping for a card draw is not always useful. (As an extreme case, consider a hand that held only 3 point cards. You can never play more than one card a turn, so your hand will always be full -- baring enemy moves that affect your hand. Tapping the deck simply draws a card to be discarded.)
Once I commit to having cost three cards in my deck, I want cost two cards to supplement them. But since cost two cards can play well with other cost two cards (even if I "waste" a power point), I usually use cost 2 cards at a faster rate than my cost 3 cards. Thus, I tend to want more good cost two cards than cost three cards. But it is a rare cost 2 or 3 card that can stand up for long opposite cost 5 (or cost 4) cards. So I do like my cost 5 cards that have great power and no waste. But these cards have two problems: psychic vortexes which prevent their play, and their difficulty making up lost tempo. Thus, I like to include cost 4 and cost 1 cards as well. (I can count at least 16 cost 4 cards I really like, so I certainly don't find them difficult to include.)
And finally, I tend to judge a deck more by "inadequate response" than "wasted power points". If my opponent throws up a giant constrictor every round, but I can only counter with underearth worms (no wasted powerpoints as an underdark worm costs 5 power), I will acquire an accumulating deficit as the giant constrictors gradually reduce my worms strength and health (until a mere 2 rounds later the worms are dead). I may replace the worm, but then there is something else that goes uncountered and thus, I have an inadequate response. On the other hand, if I counter the constrictors with aqualid hunters (cost 3, so 2 wasted power points), the aqualids can hold the constrictors at bay forever -- or at least until other factors come into play. My response is adequate. Of course, the true situation is rarely this simple, and usually inadequate response and wasted power points correlate. And I guess one could argue, that even in this case with the aqualid hunters, I should be using the 2 wasted power points to build advantage.