By how much does a vey low serial number (5) increases value of a banknote?

I'll reply to this, not because I know the answer (I suspect noone does) but because the question is interesting.

Any note is worth whatever someone is willing to pay, but I suspect the willingness to pay for a low serial number would show up in areas where the collectors are many and the number of different notes they collect are relatively few. Example - USA. The variation of actual notes are so small, that collectors look for other small details to keep themselves occupied. Thus, you get print run scarcity, fancy serials and star notes.  A note probably needs to be very cheap and abundant for there to develop such a marked.

My thinking then is that, for “smaller” fields of interest, - like collectors of portuguese notes, these won't probably fetch any extra value unless extremely fancy - 000000001 or a note with 12345678. 

Your note has a low serial, but there's also a lot of different letter combinations making the number combinations less unique.

I asked myself the same question yesterday and came to these thoughts when I noticed I had a replacement banknote from Georgia (the one in Europe) - sadly I don't imagine there to be a lot of flush georgians wanting to shell out to own a kuponi-replacement note.

Just some thoughts. 

Thanks Ernie, very insightful!

It is difficult to know for the reasons @Ernie68 pointed out.  

I know US, Canadian, UK & some Australians seek low serials but I'm not sure about other nations & the rest of the world (World collectors). I don't believe that everyone seeks low serial numbers & have bought special # banknotes from other nations for a very minor premium. I have bought sets of notes from the Philippines for about $13.00 each #1 to #10 ($130 for the lot). These are 6 digit low-value (UNC 20 Pesos) note but not 5 digit notes from Portugal.  Some collectors may prefer a no letter prefix (or single letter) to a 3 letter prefix (early series vs the most recent/common series) for that denomination.  The lower the number of digits (like 5 or 6 digit serial numbers), the less the premium. The higher the # of serial number digits (like 7-9 digits), the higher the premiums paid. And of course, most collector only want UNC (or better).

Here's a thread which discusses “What makes a note more valuable”

Hope that helps.

Generally speaking, a banknote with a number below 10 would carry a significant premium, with a substantial premium for number 1. The premium would be greater if the note is the first run of  a new series or a new design.

And grade is an  important determinant in the size of the premium.

Obviously yes, and how this info is helpful in practical terms?

Hibernia

Generally speaking, a banknote with a number below 10 would carry a significant premium, with a substantial premium for number 1. The premium would be greater if the note is the first run of  a new series or a new design.

And grade is an  important determinant in the size of the premium.

I agree with this. The note posted here may not fetch a good premium unless it has at least 6 or 7 digits serial numbers. I do not believe this note comes from the first prefix runs. 

Good luck to the owner of this note.