This matter has of course been dealt with ad infinitum and is well documented in the archives. Go to the search button at the top of this page and search for "RV6 vs RV7".
The truth is that the RV7(A) is a completely different beast to the RV6(A) and is quite different in dozens of crucial features from the empennage forward. Beware of defensive RV6 owners who tell you differently.
The fact that the RV7 has a bigger wingspan, completely different structural wing carry-through, significantly higher gross weight, larger fuel capacity, and is designed for a more powerful engine, should immediately suggest that the difference between the models is very real.
The RV7(A) is also fully computer pre-punched compared to the RV6(A) which had only prepunched skins. All things being equal, this makes it easier for builders to achieve a higher level of alignment of componentry during fabrication which (once again....all things being equal) generally results in a noticeably superior finish.
And the large market price difference reflects all of this. However the RV6(A) is still a lovely, well balanced aircraft to fly and in many cases the significant used price difference makes them very good value indeed.