As a guitar player for more than 45 years, and owner of lots of guitars (and lots of whistles), I do not agree that you can compare. Most cheap guitars (with exceptions) are pretty bad, at least the action is bad, and, as was said - there may be dull tones (or wolf tones too), and various issues that may actually be fixable: Sharp fret ends, and so on. Incredibly enough, these days, the intonation may be bad too. Shouldn’t be possible, but yes some cheap makers are able to put the bridge in the wrong position.
One thing about stringed instruments is that the wood used actually matters a lot. Not so with wind instruments, the material itself doesn’t contribute (much, if anything) to the tone. Only in how easy it is for the maker to work it. With stringed instruments the wood is the tone.
With guitars, you often get what you pay for.
With whistles, no. It’s not comparable. It just isn’t. Yes, there are differences, and yes, there are variations, but you can easily pay ten or twenty times more for a whistle that turns out to not offer much more than the one you had already. For the high whistles I own I prefer my Freeman Bluebird. Not expensive at all. I have more expensive ones. The Bluebird is more expensive than an off-the-shelf Gen D, but could I use the latter if I didn’t have the former available? Sure. Could I play my friend’s cheap out-of-tune guitar if I didn’t have one of my own, for more than a few minutes? No. It’s basically unplayable. And there are lots of those guitars.
But even the very cheap Walton ‘Guinnes’ model I bought in Dublin airport a long time ago is OK if I’m in need of a whistle. It’s not at all unplayable, unlike “cheap” guitars.
Of low whistles I’ve only owned two - a $20 PCB low D, and a $340 (IIRC) Burke. I like the latter a lot. Extremely consistent intonation, and a tone that I like. But the $20 one is OK too. I gave it away to someone in need because I don’t need it myself anymore. If it was bad I wouldn’t have given it away, just as I don’t give away bad guitars. It’s perfectly playable, and I could (and did) easily play it instead of the Burke in our little come-togethers.
When all this has been said, you can as well get one of the ‘tweaked’ ones for not much more money than an off-the-shelf one. Just get a Freeman of some model, or from one of the other not-too-expensive tweakers, if that’s more available. If there’s a problem then at least you don’t have to waste energy in thinking there may be something wrong with the whistle. There may not be so much difference compared to an off-the-shelf Gen or Clarke or Feadog etc, but at least you know that it has been through the hands of someone who knows about whistles.
Edit: As to bigsciota’s post - lots I agree with. And it’s definitely true that if you can find an instrument that somehow makes you play more, then that’s very good. But you will not find that instrument by ordering an expensive whistle on the net. It’s extremely unlikely. My Burke Viper low D is like that though - but then again I had the opportunity to try every whistle in the shop, for as long as I wished, with only myself and my brother-in-law in the shop. Something I don’t expect will happen very often. So I found a whistle that I just can’t leave on the table. Must pick it up. That’s probably good for learning to play it.. I still have a long way to go.
But the thing is - I know that lots of other people have a completely different feeling about that particular model whistle. It’s good for me. Doesn’t mean it’s good for everybody else. While a good guitar is just something that every guitar player loves to play. I have one particular one that’s truly exceptional - and would cost an absolute fortune if I were to get another one built - but everybody who tries it, without knowing its background (or anything about it), just loves it. “Best guitar I’ve ever tried!”. That won’t happen with whistles. Not consistently, anyway.