Just this week I've picked up two Red Sox game used balls. One signed. The signed one is for a 2009 CWS@BOS game where Ortiz homered to win it. He's also signed it. The other is a 2007 regular season NYY@BOS ball, not signed. Not sure what these are actually worth because I havent bought balls before but the signed ball was $69 and the 2007 ball was about the same. Works out about £90 the pair.
Also, going back about 8 years I have a Pedro Martinez signed game jersey. Cant remember what that set me back but would have been a few hundred $$$.