yup i agree that its a bit rip off by the dealer... 20,000km is considered as minor service... the only thing they change is the engine oil....
the 40,000km will be actually more than double than what you paid for the 20.000km :S:S but before you make the appointment,, just call all dealers within your area/range and you will be amazed the differences in $$ among dealers.
out family's MDX had her 40.000km major service about 4 months ago... prior the service i rang up two dealers for quote... one dealer wanted 750+ ( it could be more coz i also demanded ATF and rear diff. oil flush and change) where the other one quoted me 450!!! i won't quote the names of the dealers as it might be no appropriate to do so...