whatever you can afford, I've taught guitar for years, and the worst thing you can do is buy a cheap one, you'll shoot yourself in the foot before you start. Most parents used to say "well we did't spend a lot incase he didn't carry on." The problem with that is that some of the guitars kids used to bring, I couldn't play because they were such bad instruments. Also it has no resale value, if you buy a half decent one you will be able to sell it, if the worst comes to the worst.
Thinking more positive, a better sound will give the novice more encouragment. Ask your local guitar shop, they will sell you their best because they want you as a customer for the future.
hope that helps.