grammar - How to parse 親も辞書を買ってやったかいがあるってもんだ

親も辞書を買ってやったかいがあるってもんだよ

The context is that speaker (a child) is trying to find the meaning of a word and asks her friend if she can borrow a dictionary. The friend ridicules her. She brushes aside the insults, saying that "dictionary owners know complicated things" followed by the above sentence.

My previous attempt to translate this sentence was a disaster, so I'm going to try again. Please help me to understand my errors.

買ってやった is "bought for me"

verbかいがある is "There is value in doing verb"

I think って must be a short hand for と and I'm going to guess that there's an implicit おもう.

ものだ is one that always confuses me. It seems to be context dependent and since I'm struggling with the rest of the sentence I'm not in a position to figure out which one it means.

So here's my new attempt:

I think it would be worthwhile if my parents had bought me a dictionary too.

I don't like my placement of "too". I don't understand what も is doing attached to 親.

Maybe:

My parents also think it would be worthwhile if they bought me a dictionary.

is better. At least も is on the right place for that.

Answer

TL;DR From the given context, I guess the gist is something like:

Dictionary owners know complicated things. That really makes parents happy they bought their kid a dictionary.

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The following is just my take on this, and I'm sure there are differing views, so please take it as one of many perspectives.

“親も”

This is a common use of 〜も, where it is not really meant as a specific “also” marker. It is used in place of 〜は as a general topic marker, when simply using 〜は may seem too much like a contrast marker.

• 花子は大きくなった (Hanako has grown taller.)
  This has this risk of being interpreted as a contrast marker. It can sometimes feel like you're implying some other person has not grown taller, while Hanako has.


• 花子も大きくなったね (Hanako, she's grown taller…)
  This can be confusing when you stick to the textbook explanation of the similarity marker 〜も (e.g. 太郎は大きくなったし花子も大きくなった). Sometimes, it's not much more than a simple topic marker. It's often used when stating something as a sentiment rather than a cold fact, in combination with sentence endings like 〜ねえ, 〜なあ, or 〜よ. Here the person is not drawing similarities with anything specific, so you shouldn't bend over backwards trying to insert the notion of an “also/too”.

Examples:

• 親も大変だね Tough being a parent, huh.


• おまえも上手になったな You've really improved, haven't you.

“ってもんだ”

This is a colloquial form of 〜というものだ. Adding it to a phrase can have the effect of turning a singular event into a generalized phenomenon.

For example, let's say a teacher is pleased that his pupil has passed an entrance exam:

• 先生「がんばって教えたかいがあった」
  
  Teacher: (This outcome makes me feel) It was well worth my effort teaching.


• 先生「がんばって教えたかいがあるというものだ」
  Teacher: (These kinds of outcomes make people feel) It was well worth the effort teaching.

Another example:

• 少し休めば元気になるよ
  You'll get better if you rest a little.


• 少し休めば元気になるってもんだよ
  (It's generally the case that) people get better when they rest a little.