I haven't seen lower prices on MAX8 than -800, but I have seen that the lowest price tends to be on a 175-seater more frequently than the fraction of 175-seaters in the fleet. A lower fare is required to sell 175 seats than 143.
Revenue management sets fares based on demand. As seats are sold, fares increase. Flights with more unsold availability have lower fares.
Yesterday I received a flight change notice for a flight in August, Southwest had pushed departure back 45 minutes. When I booked, I chose this flight (800) because it was the cheapest of the day. The flight immediately before mine was scheduled to be on a MAX and now doesn't exist, presumably Southwest moved those pax into my flight (possibly among others as well). Now my flight is the most expensive of the day.
(And now I've changed to a flight with more convenient timing, for free.)
The lowest fare on any particular day is the flight with the most available unsold seats. That is likely an 800 (or MAX), but if there's a 700 with more availability it may be cheaper.