Firstly, you need to be using the same units to make comparison easier.
Now since you are talking about Velocity points and BP, I will try and do an example on this.
Say you will be fueling up 50 litres.
Say fuel price at BP is $1.5 per litre (crazy prices but it makes calculation easier).
Say at XYZ fuel station, they charge $1.4 per litre.
So at BP, you will have to pay $75 and $70 at XYZ. Difference of $5.
At BP, you earn 50 x 2 = 100 Velocity points.
Using Keith’s valuation of Velocity points (you will have to use your own), he value Velocity points at 1.2c per point.
So the 100 Velocity points = $1.2 dollars of value in Velocity points.
Since you are paying $73.80 ($75- $1.2) at BP compared to $70 at XYZ, I would personally fuel up at XYZ as you are paying way more than 1.2 cents per point. In this example, you will only choose to fuel up at BP if you value Velocity points at 5 cents per point (99% of Point Hackers will think you are paying too much).
You do a similar analysis for flybuys too because you have (or should have) a value of flybuys points.
If you want to avoid having to recalculate again and again, you should either use a spreadsheet or manual trial and error to give yourself a rule of thumb. E.g.: I will choose to fuel up at BP to earn velocity points if the price difference is say 2 cents. Or I would choose to fuel up at Coles to earn flybuys if the price difference is 1.5 cents.
Just examples. You would have to pluck in real life numbers and your personal value of points.
If you regularly spend a lot on fuel, it may be worthwhile to use one of these cards to pay for fuel to maximise your point earn. -PH link-
Hope that helps.