The big difference I have noted in the prices of tops seems to stem from obviously the quality of the material, but also how correct to original the top is sewn and seamed. They can vary a lot.
Originally these tops had their darts in the upper rear quarter area taken in with what they call a french seam, which involes a V cut, turn under each edge, and a seperate narrow piece double sewn in place on the inside. This makes for a dart that lies completely flat. Almost all of them today take the cheap and quick route by simply cutting the V, then gathering and either heat welding or stitching the dart closed. This cannot help but leave a little pucker at the top end of the dart, right where it shows worst.
Moss (Robbins) tops do this and they also have a huge rear window as an "improvement". A BT7 top we just fitted has a rear window 3" taller and 2" wider than original, which is a lot compared to original. I spoke to the Moss product development guy and they won't change their pattern.
Is there nobody out there who will develop a top truly faithful to the original and charge the amount needed to make it worthwhile?