$10k DAC in a 3k system?

While this would not be the most efficient allocation of resources, yes, a better DAC will make a noticeable improvement in even a quite modest system. 

At the Washington DC area Capitalfest audio show, a few years ago, one of the best sounding rooms had a low-budget set of components on display.  The speakers were ProAc Tablettes, and the amp was an integrated tube amp from Synthesis.  This seemingly modest system was putting out beautiful music.  The source, I believe, was CD quality files on a laptop.  Hidden from view was the DAC, which was an Audio Note DAC 5 which costs may multiples of the combined value of the rest of the system.  I think the point was to show that even a set of small speakers and a fairly low-powered tube amp can be really capable, but, it also showed off what the DAC 5 can do.

I agree that spending $10k on a DAC for that particular system would not be the best way to spend that kind of money.  I would say that the buyer would certainly hear an improvement with a much better DAC, but, money spent elsewhere would improve the sound even more. 

If the OP is planning on further improvements to the rest of the system down the road, then the purchase of a really good DAC might be viewed as a really big first step in that direction.  I can see the logic in starting with really good source components first, provided that you can reasonably assess what is a really good source component when the rest of the system is not at a comparable level (a chicken and egg dilemma).  But, my own preferred approach would be to find speakers that I really like, and then build the rest of the system around what works with that speaker (e.g., efficiency and level of difficulty in driving the speaker would set parameters around the kinds of amplifiers that should be in the system).