I have a few bottles of poppers and I will admit I am no expert on the subject. I honestly don't know the answer I just read reviews online if there are any. However what I do know is that not all poppers are made the same and this is what I look at mostly.
There are several different formulas.
Isopropyle Nitrite
Pentyl nitrite
amyl nitrate
Isobutyl nitrate
isopentyl nitrate
All these are available depending on where you live in the world. I myself have tried amyl and Pentyl and I was fine with those. I avoided Isopropyle as it can give you eye problems apparently so it put me off them.
From my experience more money doesn't guarantee better quality, but then as I said Im not expert. Buying bigger bottles is cheaper, but in the long run if your not careful wont stay fresh any extra.
To keep things fresh the only other bit of advice I can give is get another empty bottle (preferably glass, avoid plastic as poppers can melt some plastics. An old empty poppers bottle would be great for this) and use this extra bottle as the bottle that you sniff from. Add some cotton wool to the bottle and pour a bit in from the main bottle. The more your open the original main bottle, the weaker it will get as its more exposed to oxygen, hence the use of the extra bottle. Saves you money in the long run 
Good luck