All street parking is free after 6 pm, so if you can find a spot on the street, that'll be the cheapest option...even if you have to pay for 2-3 hours. I usually have decent luck over around the courthouse (Congress/Austin/LaBranch/Franklin area). There is a garage directly across the street from the courthouse (and a surface lot directly across Franklin from the garage) that should have some spots available by mid-afternoon. That's only like 4-5 blocks from MMP, and provides a decent post-game escape route, if you head away from the stadium.
There is also a garage under Discovery Green with an entrance directly across the street from the front of GRB. That may be a bit of a post-game clusterfuck, but it's worth checking out.
Of course, I've never tried to park for a WS game, so YMMV. Good luck!