what is the best, least costly DAC...?

MF V-DAC

I agree with Soulbrass. If you want something better than the Saturn, you'll need to get a much better DAC.

As for a USB/SPDIF converter, the HiFace and HagUSB have worked great for me.

low cost is subjective... i replaced a weiss minerva with a wyred 4 sound dac which i much prefer. not sure if they are in your budget, but if so, you should certainly look into the Wyred DACs.

I haven't heard the V-DAC with a PSU, only without one. If you like what the Saturn does, I highly doubt you'll be a V-DAC fan. The V-DAC sounds very dull and lifeless to my ears. It does the hifi stuff, but I view it as Musical Fatality, along with the rest of their gear.

I know it has it's fans. People rave about it and say it betters stuff that's 10x more money. I have no idea what they're listening to that the V-DAC is trouncing it. It's good for $299. It is in no way shape or form better than a Saturn. Not even close.

My Theta Cobalt 307 from around '96 beat up on a V-DAC in my system.

Everyone's ears and systems are different. Everyone likes what they like. I'm sure some people will strongly object to what I've said. Just my experience with it. Take it as you will.

To add...

Rega is releasing a DAC soon that is rumored to be better than the Saturn. Preliminary price is $995. Google Audio T Rega DAC for specs, pics, etc.