Actually, it's $250 when they sign up to adwords and spend $100. I got that a few months back, too. You don't get that much from adsense. From adsense, if you get a signup, you get $5 for when they get their first $5 in clicks. Then, you get $100 when they get their first $100.
When a publisher who signed up for Google AdSense through your referral earns US$5.00 within 180 days of sign-up, you will be credited with US$5.00. When that same publisher earns US$100.00 within 180 days of sign-up and is eligible for payment, you will be credited with an additional US$250.00. If, in any 180 day period, you refer 25 publishers who each earn more than US$100.00 and are all eligible for payment, you will be awarded a US$2,000.00 bonus. * From the adsense page itself, I also got a $250 a few months ago, was a nice surprise!
I don't know actually... I have some sites and I've placed some ads... I don't even know from which website that guy came from... Anyway, when pressing the "Ads by Google" link under or above an ad unit, is that a referral too?