Sorry for this awfully late reply. It's just that this past week has been busy.
I think I should have started this thread with these questions: "What does それを
しただけのかいがないこと。mean in those dictionary definitions? Why the past tense?".
I'm going to explain in more detail what I wanted to ask initially, since I think it might have been way to vague. Starting from this paragraph I found{
SOURCE}:
It's worth ~ing「〜する甲斐がある」
何か良いもの、価値があるものには何かの代償があります。そこでよく使うフレーズがIt's worth it です。「〜する甲斐がある」という意味です。It's worthの後に来る言葉がその価値の代償を表現します。例えば、It's worth waiting. 「待つ甲斐がある」。
The restaurant is really far but it's worth going there. レストランは遠いのですが、行く甲斐があります。
So "~する(だけの)甲斐がある/ない" means "it's worth/not worth doing...". What I've been trying to ask is: "Does [~した(だけの)甲斐がある/ない] mean [it's worth/not worth completing...]?"
And more importantly, can it refer to something being completed in the future or in general?
For example, in the following two sentences, the past form is used, but they refer to something being completed in the future or in general.
説明を聞いた後で、作業を始めましょう。
この黄色い薬は熱が出た時に飲んでください。
If した(だけの)甲斐がある does have this meaning, then it should be valid in sentences like "doing (completing) something is worth it, so [let's do that]/[I will do that]".
But now, I don't think it can be used like this. I would give a japanese example for the mentioned situation but everything that comes to mind seems plain wrong. I also can't find sentences on google that appear to have this meaning.