For a single domain, about $6 in a day. That was a great domain though that was getting tons of type-in traffic when I purchased it.
On average most domains earn me much less than that when parked though.
That's why I don't park domains that I'm planning to keep for a long time, only domains that I'm looking to flip fast.
If I'm keeping a domain then I'll develop a quick mini site on it and promote affiliate products as well as run contextual ads, then do some minimal link building for it. In the long term I do a lot better with developed mini sites than with parking.
@bigslim, that's impressive but I wonder how many domains you're sitting on to reach 5 figures in a single day from parking revenue?