I'm not sure which exact set of strings was the most expensive I've ever purchased but there is a segment of what I would call "hi-end" strings with much higher prices than "normal" strings. The ones I've tried in this hi-end segment are Thomastik Jazz Rounds Thomastik Jazz Flats and Dogal R44 Vintage Flats. I guess Optima Chromes and Optima RBs also belong to this category of hi-end strings. These are just the ones I have personal experience with.
These are nearly all "niche" strings that may not be a good match for every bass player or every style of music but they are all superb quality strings that (in my opinion) are in a completely different league to most "standard" strings out there.
All of these manufacturers also sell less expensive "standard" roundwounds btw.
However, this doesn't mean that the strings I've listed above are my absolute favorites. Like I said, they're niche strings that I reach for when I need a certain tone/vibe. My allround favorite roundwounds are probably the reasonably priced Roto 66 steels. My favorite short scale flats will always be Pyramid Gold (but I guess these are close to the "hi-end" category, at least based on current prices!).
But the objective quality of the TIs, Dogals and Optimas listed above is head and shoulders above most "standard" strings IMO. They will last forever and have an absolutely wonderful tone when you need THAT specific tone. That doesn't means they will be the best choice for all players, all of the time.
I also think there are some strings that have a "hi-end" price without really feeling like "hi-end" strings. Dunlop flats is an example of this. Don't get me wrong, they're great strings. But there's nothing really unique about them IMO. I could get reasonably close to that tone with a set of Fender 9050s at a fraction of the price. But with strings like the TIs and Dogals mentioned above there really is no alternative, if you want that specific, unique tone/feel.