If its a $6k pre, an add in DAC probably adds around $1K to the retail price - that's very much a ballpark estimate. However, the critical point is that unless you need dac / streaming functionality, then you are better spending your money on a preamp that doesn't have those features.
It's worth taking account of your long term plans for your system. If you plan any digital upgrades, then you may end up with a separate DAC and a redundant DAC in your pre, and a less good pre than you would have had if you had forgone the inbuilt DAC in the first place.