If you invested $5,000 in a government bond that pays 9% per year, you would receive 9% of $5,000, or $450, in interest payments. Therefore, the opportunity cost of keeping your money in cash would be $450 per year.
If the bond pays 6% per year, you would receive 6% of $5,000, or $300, in interest payments. Therefore, the opportunity cost of keeping your money in cash would be $300 per year.
Note that as the interest rate falls, the amount of interest you can receive by investing in the government bond decreases. Therefore, the opportunity cost of holding your inheritance money in a non-interest-bearing account decreases.