The cheaper the fabric the heavier the weight due to inferior 'crimping' of the yarns.
A high modulus cloth of lighter weight will vastly outperform a heavy inferior cloth vs. longevity vs. sail shape, etc. etc. etc.
7oz. is typical of plain vanilla woven dacron cloth for a typical 30 footer for a target venue that normally has 12-15kts.
Its all a balance between quality vs. economy .... and you rarely get both in the same package.