£2.5k will get you a real '28 if you want something trick.
If not they're just a waste of money, not significantly superior to the '21 in anyway. With enough coxing the PGM4 may be capable of better results but it's expensive coxing.
In my opinion '21's are currently the absolute bargain bike in the country. Great if you're buying, not so if your selling.
Really the only smokers I've seen fetch anything like what they should be worth are 28sp's and RGV250vj23's, everything else is peanuts.
When it's back and running nice I doubt my '28 is worth much (if any) more than £2500.
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