When you said "all seasons" I thought you were talking about the original series..
Anyway I haven't seen the new series go on sale very often, I picked up my season one box set from wherehouse.com back when they had that 25% off used coupon. I think I paid $24.95 with coupon and it came in mint condition. Since I used trade in credit, I actually got it much cheaper (probably spent less than $10 bucks on the games I traded in).
Trade in games have to be in MINT condition or they will reject them. Good current trade ins for wherehouse.com:
Timesplitters PS2 $5 at Blockbuster - $18.40 trade in
Driver Parallel Lines PS2 Xbox $9 at CC - $21.85
Stolen XBOX $5.00 at TRU - $10.35
Powerdrome PS2 $1.99 at Gamestop - $10.35
Total Overdose PS2 $3.96 at CC - $10.35
Dinotopia XBOX $4.99 at gamestop -$18.40
Bratz Rock Angelz $9 at CC -$18.40