I aim for at least a 4-1 return. ie. £1 spent £4 returned. Sometimes its a lot more sometimes its a loss.
The key is to test your flyers by putting out say 3000 each week but make changes to the flyer each week and to different areas of the same type of housing. Monitor the responce. Identify the best flyer and then go from there.
Letters are good but door knocking is best.
Mark