Personally, I have never purchased a personal house. However, I completely paid for our free company's medium house, which I believe was somewhere between 20M and 25M; I don't recall the exact amount. I also have had a total of four rooms. Therefore, 1.2M in total in that regard. My room's decoration costed a few millions in total, but the brunt of the decoration being storable means that I didn't have to pay all that much whenever I made a new room, as I never deviated from the base design very much. I have also furnished a large portion of the decoration for our free company house, but I definitely cannot put my finger on an approximate cost for it. Most likely under 10M.

Decorating is very expensive if you don't buy housing items from NPC vendors, that much is certain. I did not, with the exception of some event only items. I wish you the best of luck with your decorating plans.