The price group of a natural stone is not an indication of quality, meaning group "A" stones are not lower quality than group "D" stones. However not all representations of any given stone are good and at the very least you can be certain of no less than a "good", "better", "best" quality range. This is where you will find quality ratings of the stones within their pricing groups and those ratings are as follow:
1st pick (best representation and quality)
2nd pick (lesser representation and quality)
commercial grade (lowest quality)
Simply because someone says it's the best quality doesn't mean it is. Saving even several hundred dollars does not justify receiving a countertop made from material intended for commercial wall cladding. Natural stone surfaces are true investments and should be treated as such. The cheapest price will likely yield the poorest results. Do not buy beyond your means, but don't confuse the lowest price as being the highest value.