I already got my HD6XX from the first released so I didn't keep an eye on the price since, and I paid 199 usd plus whatever P&H it was. I remember I got an email back a while ago (easter sale may be) from A2A that they got the HD6XX and HD58X on sale... and I check back this morning it's back at just shy of 400 for this Co-lab model.
If you were looking at the 660S which is supposedly newer/ improved successor to the HD650 then that is now selling for 600+
To say should you get it or is it worth it, I don't have an answer to that.
Sure, if you wind back the clock, about two decades ago around the Y2K, the HD650 could may be the most sort after headphone, along with the DT880 (premium) Edition and AKG K701, these three collectively makes up the " holy grail " / trinity of the headphone summit. A lot has changed since, heck I lost count if you ask me how many prime minister has changed baton, personal audio has taken off, smart phone replacing DAP, newer material, newer driver technology etc ...
This HD6XX, for a 199 I paid, is alot of headphone, it's fun, engaging, popular and piece of history (being it based on HD650), is this my endgame headphone? Not exactly. Did I regard it? No.
(Off the record, if you are after the HD660S and jump on YT, look up all the popular channels and reviews, the rumours is there is a different model which drop/ A2A also sells and after modding they will sounds almost identical. )