Set yourself up a garage http://www.mygranturismo.net/ you can add tickets through items section and then click certain tickets to see what you can get.
IMO most desired would be 1967/1989/1992/2008/2009/2010 level 17/21/24 and also the 1000 ticket
I would add that ticket value is also a function of what car you want. If you really want the C5R you can get it with a 2000 ticket or wait for it to show up in the dealership. If you really want one of the Ferrari F1 cars, get a level 24 ticket.
