When discussing the contribution with the dealer I would stick to my guns in that it is his responsibility to fix this - you're not saying it's his fault and you're assuming that he was taken in too. But the law says it's still his responsibility.
It's not unreasonable for him to ask for a contribution if it's a case of 'betterment' in other words if he can only get an engine with 30,000 miles on it you're ending up with a car much better than the one you paid for and it's fair then that you should contribute.
What you're not doing is 'splitting the difference'
You see from that ebay item that a replacement's not all that expensive it's the labour to fit it that costs money - a friend of mine recently had an engine swap on a mazda which was about £600.
Obviously though if he's running a workshop or the larger organisation is that cost to them is a lot less.
He might be trying to get an engine cheap - say that one on ebay for £150 paying £400 for a fitting and then telling you the lot cost £1,000 and saying that you'll have to pay half.
Ask for the paperwork on the replacement engine - you want to know its mileage and evidence it cost what he says.
Note the cambelt on Zafira is 100,000 miles
http://www.mytimingbelt.com/Results.aspx?ModelId=519
If the replacement engine hasn't had that replaced and it's that sort of mileage you want that replaced before it goes in the car ! much easier then.
Failed cambelts kill engines too!