I snagged a used pair recently for $300 USD, Amazon has them for $400 new. Headroom lists them at the full retail price of $499.
The Fiio E17 and E09k combo reportedly pairs well with the HD650 (I'll report back once everything arrives and I have the opportunity to listen to it). The bonus is you also end up with a portable amp/DAC which is great for driving multi-BA IEMs (no noticeable hiss with the UM3X, which are VERY sensitive to amplifier hiss).
Edit: I just noticed that the HD600 and HD650 are the same price on Amazon currently
Yes, I have that combo and I'm very pleased. For ~$250, it's really a solid performer and gives me a portable solution for my SE 425s =D. The E17 is a solid DAC too so even if you upgrade the amp in the future, you'll still have a decent DAC.