Leveling BNT quick question

[RyogoKusama]

If you're consistently near the leves cap (which is 100) or you're simply not using them much, then definitely use them for BTN, but if you're leveling a Disciple of the Hand too (crafters) I would suggest to save the leves for them, it really saves a lot of time and money, since higher level craftings require more quantity/quality items.
In the end, with a gatherer like a BTN you can simply grind some experience up collecting items or simple old-fashioned gathering and make a profit out of it (which is kinda the whole point of the class = having crafters selfsustain and/or selling materials).

About Collectibles, it's easier than it looks, the GP range is there to make you understand how many abilities you're able to use to enhance your collectible rate, but as it has been mentioned already, it doesn't necessary means that you obtain nothing if you're under a certain threshold, you will simply get 1 or 2 items less or items with a lower collectability rate (of course, this strictly depends on the different nodes) thus receiving less exp/scripts.

Also, one final big suggestion, I don't know if you know already or if the guide you've been following mentions it, so I apologize beforehand, but it happened to me quite a few times to see people using "Toil Of The Pioneer" to discover un-reachable items inside a node, when try to gather from special time-nodes... well, that is a complete waste of 300 GP imho!
By now, every website/database oriented about gathering tells you on which specific slot you will find a certain item (like slot 1, slot 2, etc.) this means you can simply use a gathering attempt (ie: simply throw a hit) in that specific slot and you will un-cover the item you're looking for, then you can activate "Collector's Glove" and use your Appraisals/Collectable abilities to reach the result you want: in this way you lose a gathering attempt, but you save 300 GP (which is way better).
Of course, if you have never gathered that certain item, after the initial hit you will uncover it as unknown at 25% rate like always, so you will probably waste that node at this time just to "unlock it" for future gatherings.
Once you hit a certain level range, I suggest you to take some time and go identifying the different nodes so that when you're gonna need them for something, you will not waste your chance (obviously, I'm speaking about timed ones, but this works for persistent ones too)!

I apologize for the wall of text, hope this helps!