I would not use much time thinking about this...
As my experience over the last many years, has been that newer lines are of an extremely high quality and they last almost "forever".
Only accidents or if you are a "dog" with your lines, can make them wear out fast it seems.
Some might say that "some" lines are better pre-stretched than others.
This might be true - but on the other hand, I've seen some new line sets from different suppliers, that works great most of the time, and then suddenly there can be one lineset where a line will "give" a bit too much...
So even here, nothing is "safe" regarding this very issue.
New lines will stretch quite fast though and typically in pairs, so you adjust accordingly, and end of story
Must say I am REALLY REALLY amazed by the quality and life-time of kite lines today
Compared to what we sometimes saw back in the days - yiaks
So I would not worry much about this - just go and get a new lineset
And there would most likely not be any relationship between price and quality, if you take the lines typical kite brands use today - as many lines are quite equal even when the price is not
But SOME lines might be more different, true - it is just most likely at expense of something else then, like thicker (more drag and slack) or heavier etc...