actually, pretty much any visa can be extended, but multiple entry visas often loose their multi-entry status when extended (maybe that's what you were referring to) unless you pay an agency to do the legwork for you (which costs around $200).

also, i can vouch from personal experience that extensions can be processed as much as 7 days after expiry. but, this often entails a visit to the back room and a power trip style lecture.

i've been in the above situation three times and have never had to fork over extra cash. however, the official penalty is around y400 per day iirc. on a related note, one of my friends from japan overstayed her visa by a whopping 60 days about 5 years ago and learned her lesson the hard way at the airport when she tried to leave. the officials didn't charge her the whole $3000, but they did hit her up for y5000 ($600). because she was a student at the time, she didn't have that kind of money on her person, but a fellow traveler did and advanced the cash so she could get out of town.

while i'm in story-telling mode, i might as well divulge my personal favorite. several years back one of my colleagues stayed in a philippines a week longer than he was supposed to. while the airport guys were trying to figure out how much money they could squeeze out of him, he decided to take advantage of the chaos and simply walk on through. he proceeded directly to his airplane, which was already boarding and hid in the bathroom until after they were airborne.

i certainly don't encourage people to overstay their visas, but if you do, it won't be the end of the world.