We did a 5x10 addition as part of a much larger addition. By far it was the most expensive part. Every addition requires excavation and foundation work, and much of the cost involved is getting the equipment on site. So yes, an 8x10 addition (or a 20x10 addition) form a cost perspective is not that much more expensive.

Adding a second story isn't that much more expensive, and might make more financial sense than a very small addition.

we have a small house on a small lot and really don't want to put an out of proportion addition in the back to expand our small kitchen, but from what i gathered the costs of a 8x10 might not be much more than a 4x10 but it adds much more value/appeal. i'm fine with a 2-story 4x10 but i know that's even more expensive.

so, does it really not make much sense at all for smallish additions?