Why is uncommon geometric houses expensive to make?
Your specific questions are nearly impossible to answer, partly because a question like "how much does a house cost?" is best answered by "well, how much money do you have?". There's also no such thing as a "regular house" though I'll assume in this context you mean one built using rectangular shapes. :)

It essentially boils down to the fact that anything non-standard is generally more expensive. 

Designing a house from scratch requires more of an architect's and structural engineer's time than modifying an existing design, and likewise, changing the location of non-load bearing walls is significantly easier and cheaper than moving load bearing walls (where a structural engineer needs to recalculate if it will work) or moving the locations of things that require lots of mechanical infrastructure like the kitchen, bathrooms or furnace. 

You're going even farther, and requiring designs and calculations that wouldn't be needed (or are already largely figured out) for rectangular shapes.

The building permits department and inspectors are also going to have to do more work, because they need to verify everything.

Most construction crews are also quite experienced building rectangular shapes, and not weird non-standard angles. 90° angles are easy (in fact, all carpenters carry multiple tools that can easily and perfectly create/verify 90° angles). A heptagon involves ~128.571° angles, and no doubt there will be additional strange angles throughout the rest of the house. Even if you found a crew that was experienced building with that (probably not), it takes longer (meaning more $$) to measure/verify/etc than it does square corners. With the crew inexperience comes greater uncertainty and potential for problems, so the cost goes up for that, too.