ymribiat, i think RebE’s Rebbe answered the question – if you are in limudei chol and not doing anything, this is Bitul Torah. Does this answer the question?
Extending this approach makes it difficult for others:
– if one works well beyond what he needs, is it bitul Torah?
– to what degree you need to build up 401k or should you just have bitahon?
– if you eventually need to start working at low wage, is it bitul Torah that you could learn to do a better job and work less hours?