I think you'll have many that see 650 tickets and 20 quid a ticket, dont factor the 100 free, and are coming to the conclusion that you are collecting 13k for the Boxster S.
Maybe if you could have started with an Evo, Scooby or the likes of might have grabbed more attention.

What about writing off the 44 free remaining tickets, so essentially 88 tickets. and reducing the total number of tickets and trying to go with a 500 allocation?
The people who are in already are in for 2 for 20 and should be please to see their chances improve further.

Unfortunately to get something like this off the ground, there is probably going to be some losses incurred until you can build the big customer base and can turn over cars at a rate of 1 a week.