We gave our kids strict instructions for souveniers on our last trip (and are doing the same for our May 2010 trip).
They were allowed ONE
lego kit under a certain dollar amount from the Lego store (favorite place!!!)
We paid for 2 tshirts for each child - beyond that, they had to use their own money. This year we may have to adapt that rule, since there is no way DD will leave DL without an authentic Princess costume! So the boys may get an extra tshirt or two out of the deal!

They were each allowed to purchase ONE "stuffy" (mostly for little dd who lives for stuffies!)
They were allowed to pick ONE other toy/item, but they were not allowed to buy it until our final day at the parks (to ensure that they didn't purchase the first thing they saw, then get sick of it and want something else by the next day!)
We weren't concerned so much with how much money they spent as with how much stuff they bought - we didn't want to pack a whole bunch of junk to haul back! In the end, they all picked the same toy, so it worked out fine with respect to "equal value" purchasing.
Best of luck - the marketing experts at DL are indeed experts - the kids are inundated with "buy this and you will be happy" advertizing, so we spent a ton of time discussing purchasing with the kids beforehand. We also had a policy of not "shopping" until the end of the day - by that time, they were too tired and just wanted to go to bed! I know, I am an evil parent to deprive my children so!!
