Things have probably changed some from the beginning of the program. It was a long time between proposal and legislative approval and then implementation.
At first the requirement for income was US $1,000 per month from a company pension and/or social security ($12,000 annual deposited in a Belize bank to be verified once a year by BTB) OR US $2,000 per month if the source of income is from investments only ($24,000 per year in a Belize bank.)
Now, Belize being Belize and having to include in all plans the "Belize Factor" the final, final requirements are always subject to uncertainty.