A quick look said you could park for a fiver - need to look in more detail. I just know from going to the veledrome a lot its a pain to get home late from there to Brighton. We often park at Stratford International but the crowds clear a lot quicker after the cycling than I imagine the will for...
I may go to Ebsfleet International - train takes 12 minutes from there direct to Stratford International - thats about 10 minutes strool to the Olympic stadium. Looks like 4 trains an hour.