ok, i have some numbers.
total lease payment a month for $20k over 58months = $838 p/m (before tax)

impact on take home pay = $575 p/m

take out the stuff i would be paying anyway (at the moment after tax has been taken out) of fuel $220, rego $58, comp insurance $43 = $321 p/m

$575 - $321 = $254 p/m for the finance + balloon of $4000 after 58 months

$254 x 58 months = $14732

add the $4000 balloon at the end and the $20k has cost me $18732!

seems like a amazingly good deal to me - or what am i doing wrong?