What is considered the normal difference in elevation inside a house? I've heard it can be as much as 1/4 inch every ten feet.

Nothing is "normal" in houses. Joists deflect, so floors will be marginally lower in mid-span that at the supports. That's why good carpenters frame the joists with the "crown" up, hoping that they settle near to level under use. A foundation may vary widely in elevation. Block foundations are normally better than concrete ones, because the masons can correct as they go. Footings should settle not at all under normal conditions.

If a house can be off 1/4-inch in 10 feet, that means a 40-foot house can be off an inch. If so, it's pretty sloppy construction, or something not good is happening. It should be migty close to dead level.