How do you buy a 100K house? You cheat. You can use the familyfunds cheat if you have Pets, otherwise you stick the sim/family on an empty lot, and then motherlode (50K) and kaching (1K) to your heart's content, and then move them back into the sim bin prior to placing them in the expensive lot of your choice.
Surely I'm over looking something obvious or there would'nt be expensive homes.
Yes, you are overlooking cheating, such as actual game cheats, moving in new sims just to get their money or moving out family members individually and reassembling the family on a new lot as described by miros.
And I'll echo miros' "why can't you move the family out?"