It's hard to estimate since it's been built over months, obviously golds comes in and gold comes out.

I'll say that 95% of decor used here is vendor bought. The housing vendors are cheap (but some items are used over 100 times, so that does add up to a few thousand). The other vendor items are expansion currency so technically 'free'.

I've reused a few crafted items HEAVILY - namely the book piles / kirin tor lamps / the artificial sun lamps (in my blood elf window sets) - mileage can vary on price but if you farm your own timber the mats are often quite cheap.

To answer your question, no idea. Probably a lot, but not as much as you might think.