Section 7. There shall be a limit on the estimated amount of grants approved by the authority during a fiscal year. For fiscal year 2008, the limit shall be $500,000,000. For each fiscal year thereafter, the limit shall be the limit for the previous fiscal year plus the lower of the rate of growth in the dedicated sales tax revenue amount, as defined in subsection (a) of section 35BB of chapter 10, or 4.5 per cent.