Ok, so this is just a fun one I found by accident...
The USS Enterprise in Kre-o!
Looks like a pretty neat set, and reviews seem pretty good, though apparently it's a bit of a pain to put together.
The nacelles and deflector dish light up, and you get Kirk, Spock and McCoy minifigs.
432 pieces, so about 7p per piece, which seems pretty decent.
Current Amazon.co.uk price is £39.99 and eBay about the same or more, so this seems like a good saving.