in Ontario: Donation <=$200 get 15% from Federal + 10% from Province credits. So $200 donation save you about $50 incomeTax (Your real donation is $150).
Above $200 (in 2007 year return, before is $400 if I am right), you get your income after RRSP tax maximum rates return saving (like 42%, etc...) for the Toal$ -$200 part. So $1000 donation may reduce tax $420, your real pay out is $580.