That really depends on how long you have been sending the joins. It should go up the longer you are with the program.
The start of the 4th month should start to show good returns, by month 6/7 you could see the joins starting to be as much as $40 on average. The price of the site makes a difference as well.
$20 isn't bad.. If you are converting lets say 1:100 and when you advertise someone else that pays $35 and you convert 1:200 then you are doing good
