It really depends on many criteria. For example, if you find a lower cost rental, and your income is very low, it's possible for section 8 to cover the whole thing. At one point I had an apt for $850 and I not only had it all paid for but they gave me $67 a month to supplement my income. It all depends on a lot of variables.
This varies by the fair market rental value within your specific city. The voucher will be based on your monthly income and will pay anything above 30% of your adjusted monthly income up to a predetermined amount. For example, if your monthly income is $1,000 a month, and you want to rent a property for $500 a month, you would pay $300 a month in rent and the voucher would pay $200, as long as the (Fair Market Rent) was equal to or greater than $500.