I've read that the HD 650 hold their value well and that second hand pairs in good condition don't fall far behind new pairs - price wise. I'd check the 'for sale' section of head-fi to get a feel for prices. I don't know where you're based, but in the UK, around £250 seems to be a good price for this headphone - brought new.
If you do plump for second hand, watch out for the colour of the diaphragm. Older pairs had a dark colour diaphragm and the sound is supposedly darker and more 'veiled' by comparison to newer versions with the silver colour. The diaphragm is visible through the outside cup grill - there are plenty of threads on this subject with pictures.
The thing to bear in mind - as previously expressed - is that in order to get a taste for the true potential of the HD 650, you need a high-quality and powerful amp. Therefore the outlay for a good HD 650 setup can be considerably more than the headphone itself.
You're in a good place right now in that you've had a taste of what a good headphone can deliver, but since you don't have the HD 650 anymore, you're free to explore and research all the options. There are other great sounding headphones out there which aren't as hard to drive as the HD 650 and are therefore easier on the wallet. Or, by reading up, you can determine what you need to get what you want out of the HD 650 and start saving.
This is the approach I took and I'm so glad I did. To my uninitiated ears the HD 650 sounds good out of my Little Dot I+ (a budget, hybrid tube amp), and were I none-the-wiser, I would be satisfied to stay with this sound. However, when hooking the HD 650 up to my Little Dot MK IV SE, which is an entry/mid-level OTL tube amp I purchased specifically to pair with the HD 650, they sound completely different. The sound is so much more engaging, alive and realistic and brought me surprisingly close to being at the live performance - which to my mind is the ultimate goal.
There are lots of reviews of the HD 650 paired with a whole range of amps which describe the experience far better than I can.
Good luck and let us know how you get on!