Raybyrnes– I don’t think your example holds water. Their monthly nut on a $250K place was probably around $2000. The property tax and HOA alone in Del Sur is probably going to run them around $1200.
Assume they net $350K on the old place, they’re still going to need a loan of around $400K for the new place. Let’s just call that $3000 a month. Are you sure you want to stick with their payment isn’t changing much?
I’m not saying people aren’t doing this kind of thing, but don’t kid yourself that the “move up” buy makes financial sense in a lot of cases.