Well, this is very difficult to answer because Wilson usually pays for House's lunch whether he knows it or not. He occasionally wonders where all of his money is going.

5. When Wilson, on behalf of House, paid a lawyer's bill so unspeakably gigantic that he couldn't afford to for the next few weeks.
4. When House had somehow got it into his head that Wilson was having an affair with Cuddy, and Wilson, noticing the familiar obsessive signs, decided that it would be best to avoid him for a month or so.
3. When House, bored, turned Wilson's office upside-down in a hunt for evidence of something to pester him about, and Wilson refused to pay for his lunch because he was too angry.
2. When House stole his wallet, curious to see how he would react and how long it would take him to suspect House.
1. When Wilson was suddenly and mysteriously taken ill, and House was so busy trying to work out what was wrong with him that he forgot to eat.