How do I Decide to Sacrifice or Preserve "Smaller" Naval Units  (E.g. Subs) In Axis and Allies?
After having left a "partial" answer to try to frame the discussion a week ago, I'll now try to finish it.

In purely NAVAL terms, a sub is worth more than 8 IPCs. Specifically, a 24-IPC battleship has four attack and four defense rolls. So do two subs. On this basis, a sub might be worth 12 IPCs, about the same as a fighter. And more than a transport PLUS an infantry.

A sub also has "first strike" and "early withdrawal" capabilities that battleships and fighters do not, adding to their value.

Unlike the others, a sub cannot fight air or land units. You could say that diminishes the value of the sub, or at least that this balances out the sub's special capabilities.

So if we're valuing the units for their purely naval capacities, there's probably nothing better (for the money) than a sub. The reason to use the sub as "cannon fodder" is if we want to save planes or battleships for non-naval functions like land bombardment or anti-aircraft. And that these needs took priority over what was going on at sea.