Right now the HDTV manufacturers group is trying to get the date pushed back to 2010!!! Because so far fewer than 100,000 HDTV and HDTV-ready sets have been sold, or so I have heard. That's about 20% of the number they hoped to have out there by this time - hence the effort to push back the all-digital date. Not to mention the MPAA and others are trying to force a copy protection scheme into the existing standard - which would render many existing HDTV sets obsolete!
Anyway, supposedly those digital->analog HDTV converters are supposed to be under $200. But then again, by this time HDTV sets were supposed to cost only $500 more than a standard set. And we know that's not quite the case. It's more like a $1500 premium at the moment. No one makes or has even announced a converter like you are asking about, since there is at least a 5 year window before anyone will need one. So it's hard to predict what they will actually cost.....