Proceeds of disposition is the amount you sold the property for. If you sold the property for $500,000 then your proceeds of disposition are $500,000.
You are also going to be asked for the ACB (Adjusted Cost Base). This is the amount you purchased the property for. It would also include the legal fees, tax transfer costs, as well as the costs of any renovations (upgrades) you made to the property.
http://www.cra-arc.gc.ca/capitalgains/