Thank for your answer, Alysia! Though I read your answer after i sorted it out. For people who's got the same problem, here's what I did:

I went to the PSB Exit & Entry Administration (Yonghegong subway station) with low expectations, especially because last time the staff behaved like if not a-holes, then at least jerks. Especially the duty officer in desk 38, who didn't speak english and simply refused to talk to my chinese-speaking friend on phone.
Today my experience was completely different. I went straight to desk 38, which is the desk you should go to with urgent errands (bigups to the embassy for that tip). The girl serving as duty officer was terriffic (badge number 013289 by the way. If you meet a grumpy older man instead, consider going back next day). I explained my situation sincerely and spoke slowly. She spoke and understood english good enough to give me the right papers for extending my visa for the few days i required. Apart from the regular visa application form I needed a form for urgent applications, where I spilled my heart out and told them how sorry I was and all that (here, just use your best language, be polite and truthful). Went back to the nice duty officer with the two forms plus a copy of my flight ticket. She eyed it for a while, then told me to get in the regular line to the visa application desks. After I handed in papers and passport, I was done!

R-E-L-I-E-F

Oh, didn't think of asking, but I assume it's cheaper then the 1500RMB+visa trouble in the future.

But who knows?

I mean that I was so happy that I forgot about the fee, which is surely more than the usual ≈160RMB, perhaps the double or likewise.