If you inspect the rendered Grid, you'll see that the btnExport ID is actually much more nested because it is rendered in a template container. That makes it difficult to target with this syntax.
A better approach may be to simply use the RadGrid as your PostBackTrigger. Due to event bubbling, the command fired from the command item will cause the Grid to trigger your postback event. You can then handle the OnItemCommand grid event on the server and use a CommandName to detect when your export button has been clicked. Check out this page in the docs for more info on command bubbling:
Hope this helps.