The DataGrid doesn't currently support key paths, it just supports the accessing of the properties on the base "data-object". We've had this issue raised before and we are planning to add it at some point in the future. Until then, a neat workaround would be to create an object specifically for the format of the data grid (one property per column on your grid).
@ interface GridDataObjectPortfolioTile : NSObject
@property id tileLastPrice
Then you could simply create a set of these GridDataObjectPortfolioTile objects, populate them with the data you need from your PortfolioTile objects, and then supply these new objects to your grid instead. I know it isn't a perfect solution but it means that you don't have to change your current data structure.
Hope the above helps,