I know it's pretty obvious why, but here's some clarification regardless, from Kojima Productions' Kenichiro Imaizumi:
http://twitter.com/kojima_pro: (he's referring to recent Japanese rumours, which stem from finance issues, if I recall correctly)
"Recently in the news, it's been written that MGS4's development costs crossed 5 billion [yen], or 7 billion. It didn't cost that much. If it did cost that much, it would have been multiplatform. But, of course, the data can't fit."
Here's Kojima-san's Twitter, for anyone interested: http://twitter.com/Kojima_Hideo