The spacing of sails may vary a lot depending of the intended use and type of sail.
Some wave sailors may go 4.7, 5.0, 5.3 etc. while I know formula sailors who will sail their 10.7 until they can go comfortable on a 6.5 with slalom gear.
For everyday freeride use my best experience is to move upwards in 20% intervals from the smallest sail. This will be like 5.0, 6.0, 7.2 etc.
Again it depends on the intented use and the board. You may use two different boards for the sail range.
Modern freeride sails have a huge range though.
Regarding the price I have no experience of a higher price meaning better quality at all unless you are a top competition sailor... Materials are almost the same and they're all made in labour cheap places in China...
Some brands use a lot of time on renewing design (expensive) ex. NP, while some are more conservative (cheaper) ex. Tushingham.
I've had both NP sails and Tushingham. Tushingham are about 15% cheaper, and to me, their durability is way superior to NP. Performance is excellent on both brands.
Just my personal experience though