You can buy tickets on the gate.
£155 for Fri, Sat and Sun
£50 for Fri
£70 for Sat
£90 for Sun
I'm going on Fri (practice). I go Fri every year. Obviously, not as exciting as race day, but still lots of fun and great to see the bikes come screaming by - even when they're not competing. Pretty relaxed atmosphere. Not all the stands are open. I like standing at Village (the stand is closed on Friday's) and sitting at Luffield (the stand is open). But, Luffield fills fast so by 1100h there are often no paired seats available. No problem if there's just one of you.
Pre-purchase tickets were £30 and parking free (even for cars) on Friday. Parking is free for bikes all weekend (I think) but cars are expensive.