I have no idea how much it costs, but may be cheaper than 29 EURO. Then again, it could cost you more.
Best bet is exactly that - it's about 10 Euro (less if you can find some people going to Kostrzyn - people will approach you in Lichtenberg asking if you're going there - if so, it'll be about 6 Euro) to Kostrzyn, then about 25zl from Kostrzyn to Poznan. All in, about 15 Euro each way.