Too many variables to generalize, but its possible to get one of today's great DACs, for less, than it was to get one of yesterday's great DACs. And today's best DACs, in my opinion, are unmatched. Something like the NAD M51 or Mytek 192.
But if you're comparing a used TOTL DAC, something like what is in the old Denon DVD-5000 I have, to one of today's cheapest DACs, you're very likely better off to stay with the older DAC. The analog output stage, power supply, etc, is often better and it swamps the other advancements (my opinion).
In general (again, hard to generalize), I think digital has come far enough to not even consider older players/DACs. Go in for something decent today and don't look back. Digital has gotten REALLY good.