Either way it's a negative flow of cash. I know you can't rebuild a 4 stroke top to bottom for $1,800. I also know that you have more engine maintenance on a 2 stroke. Much like what Hershey said. I can't justify buying a new bike every couple years because I seem to not ride as much as I used to.
I do a top end typically once a year. Costs about $180 give or take. Oil changes every other ride usually and an air filter I try to get every ride depending on dust.
I agree that it's easier to ride the 450s and there's a lot less stress on the motor. I've only ever ridden a 250f a handful of times but I felt like I had to ride like barcia (probably because Im slow). Not sure what maintenance is like on those. I've heard horror stories about people blowing up 4 strokes and it costs more to fix them than what they're worth.
In the end, any bike you buy is going to cost money in maintenance. Pick the one you have the most fun on and go ride.