It really depends upon the horn.
For instance, I personally do not like older american horns due to the keywork and especially the table keys.
I'd rather go with some Dolnet french horn than a US horn at that low of a price range. Or even an Evette (french made) as I do like the Evette keywork.
Buy I know many prefer the vintage US horns.
As to where you draw the line?
Do you want to pull it out of the case when you get it and play it right away, or have to put time, effort and money into it?
and then It depends upon how much work/money you want to put into it.
Does it require just a few pads or a total overhaul ?
Is the body clean with no major dents or does it need dent work ?
Does the looks of it make you green with envy or it is literally green with envy ?
There are people who will buy a horn and repad it themselves to save money, others are not able to to that.
So it all depends upon what "line" you are talking about.
personally I'd rather spend $2000 for an SBA that needs everything than $7000 for one that is ready to play.
but lower down I'd rather spend $1000 for a Couf alto that is ready to play than a $400 one that needs everything.
So for me it all depends.