The sienna has an 8th seat option, but I'm not sure when they started that. You might not be able to find one in your price range, since they retain their value.
The oddyssey also has the 8th seat option, but it is pretty small and I don't think many seats would fit there. It is really only good for older children, as an adult probably won't fit there either. Again, I am not sure when this option came available and you might not be able to find one in your price range, since they retain their value.
The venture/uplander/montana have an 8th seat option, but I am pretty sure the crash tests are not great.
The suburban has 8 seats and there may be some SUV's with 8 seats, too.