Think about how much more maintenance/parts/labor etc you'll spend on a beater..... what about the difference in gas expense?
15k miles year = roughly 440 gallons of gas in the XB
15k miles year = roughly 682 gallons of gas in a beater @22mpg. Just in gas ur saving $450-500 year or roughly $40/month with the XB. If you factor everything in the XB is probably only about $150-200/month more than a 10yr old beater given gas, repairs, maintenance, etc.

I'm closing on my house in April so I know where you are coming from on ways to cut costs. You could probably trim half of that monthly difference just by changing a few small things. Don't go out quite as often..... if you eat out often, try cutting back on that... big savings there. If you had a $500+ car payment you might have a point, but not the case here.