check www.mygranturismo.net and it will tell you the costs, levels, bhp, everything of all cars and whether they are tradable or not, its a very handy site
the trade limit is 950,000 but that is brand new price, so a UCD car that is in the high 920,000s the new price would be pushed above 950 which would make them untradeable, i think the general rule for used is if its over 920,000 ish you might not be able to trade it.
a new premium car is only tradeable if it costs max 950,000.
The same rule applies to the used cars, that their "brand new" value had to be less than 950k, stupid convoluted rule but thats the geniuses (or is that geni-i?) at PD, taking the fun out of GT5 one update at a time