We need a two metal men, a gigantic green CGI monster, a Norse God, several big name actors the likes of RDJ, SLJ, Edward Norton (hopefully,) ScarJo, Chris Evans, etc. etc. etc.
Really, realistically, I just don't see it being done for less than $250 mil. If Spider-man 3 cost what it did, I could realistically see Avengers falling within the same budget range, even with them saving money on Whedon. Marvel is skimpy though, and they may want to pull this off for $200 or less (though it's kind of hard to call this skimpy...) If they can do that, all the more power to them.