Kordeth said:
All characters get the same number of power selections every time they gain the appropriate level--"losing" a class power in this instant is absolutely no different than choosing, say, Cleave instead of Tide of Iron from your own class's list of powers. You aren't losing anything, you're making a choice. A choice, remember, that retraining allows you to make even as a single class character.
The opportunity cost of something is equal to the value of the next best choice you could make. So, if your two top choices for the one power you can select are Cleave and Tide of Iron, and you very slightly prefer Cleave, then the opportunity cost of selecting Cleave is Tide of Iron. That
is a cost. You could have the benefits of Tide of Iron, and you are giving them up for the benefits of Cleave. Tide of Iron's benefits are what you're paying for Cleave.
To make it really explicit — in 3e, your 19th-level wizard, upon reaching 20th level, could take a single level of cleric. The opportunity cost of that was giving up a 20th level of wizard. You clearly lost more than you gained by making that choice, didn't you? That loss was opportunity cost. Yes, you're still better than a 19th-level wizard, but that isn't the basis for the comparison; the comparison is to what else you could have done with that 20th character level.
Less extremely, consider a 20th level 3e wizard in combat, who is down to two spells –
meteor swarm and
fireball. The cost for casting
meteor swarm this round is giving up the damage from
fireball this round.
You're playing chess. You could move your rook to safely capture your opponent's knight, or you could you could move your bishop to safely capture his rook. If you take the rook (the more powerful piece), your opportunity cost was his knight.
You want another class's power in 4e? Then you're paying the explicit cost of a feat and the opportunity cost of the best power in your class you could have chosen otherwise.
The question then becomes, will another class's power generally be worth the combined cost of losing a feat and the best available in-class power?
If the powers of the classes are finely balanced enough that the best available out-of-class power doesn't make you more powerful than the best available in-class power, the answer will be no; selecting a cross-class power will always be a suboptimal build because you gave up both an in-class power and a feat, and in exchange got a power only as powerful as your in-class power.