Hmm, yes it is a tradeoff, if you really want ultra quick and small pack the GoBike, Brompton, Birdy (though i really don't like the Birdy too much personally) and 1 or 2 others are pretty much the only options, but they are rather expensive as you said.
Suitcase packing is just generally difficult to achieve without significant dissasembly. Very likely some of the smaller wheeled folders from Dahon and others would fit into a large sportsbag or smallish crate with only the folding (not dissasembly) rather than a suitcase. The sportsbag or crate would still be airplane 'legal' i think and fit in your cartrunk. If you do consider the sportsbag route i think the Downtube could be an excellent choice at it is very affordable and comes with bag.