Below is my personal opinion:

Longtime ago, I saw the 3 volumes of books,
I cannot remember the exact title of the books (maybe in French?), 
It kind of volume 2 describe some OO languages,
Volume 3 describes some scripting languages.

In the scripting languages, the book even mentions something for
music scripting languages. That surprise me a lot, I never heard a
computer language for music. (I did see some for graphic image and
animation)

I think when you try learning a new language, it is like Dave said,

>  Learning a language means coming to understand it: the philosophy,
>  the idioms, and the "way things are done". 

Some language is designed for some specific domain, not all languages
are
general purpose, even they are general purpose, and other language may
suit
better for some problem domain then those languages.

I think the important is you learn new language that will *open* 
your mind to view/thinking stuff.
It kind of "there are just an other better way to do the job".

It likes sorting a list, there is just a lot of different algorithms
to sort a list. (Do you think you just need to know one, and that suit
in
all situations? If yes, all you need to know is assembler language :-)

In case of "cost/reward", it is difficult to estimate that.
If you base on reward, you should specialist only in the language that
is used
in your current job (or future job). That's it.
I really not idea about this "cost/reward", give you a similar case.
We learn
from school physics, chemistry, ...etc, may-be now in your current
job, you
just need computer science and some math; can you say learn calculus,
physics,
chemistry ...etc are just waste time? (If so, why we still teach our
children that stuff? If the child just want to know computer science)

I remember longtime ago, I tried learn Perl, my co-worker ask me
what's that ?
After I explain, he just told me that I am waste time. Well, I never
have a job
using only Perl, Perl is just like a tool help me a lot. And it also
gives me
some new view on regular expression (the no-greed invention).

Same thing for Ruby now, I do not have job and it also rare to find a
job that
using Ruby only. I still like to learn it and have fun :-) 
(I will never regret learn Ruby. Am I waste time again? Some of my new
co-worker said so again. I hope not :-) )