The very few times I did any kind of designing/decorating for a fee, I always left it up to the recipient what they paid me. Frankly, if the recipient hadn't generated great wealth in the game, I didn't want to put them in the 'poor house'.
Since no price was decided upon or suggested by either party, I would suggest paying close to what you feel the design is worth to you and your enjoyment.
These responses should also be a good help for gathering a starting point. Again, as a very small time designer, if you've only accumulated a mil or 2 accross all characters, I wouldn't want to to pay me more than 30% of that. But that's just me.
Good luck with your house and payment!
P.S. Let us know how it turns out. Especially if you are really happy with the result! If you are really happy with it, feel free to post pictures. That may also be free advertisement for the designer, but you may want to get their permission to say their name.