Most likely cheaper than erecting scaffolding!
Most of the cost of a drone flight is in the planning rather than the actual time the UAV is in the air. The price will depend on the location of the roof that needs to be inspected. Days of planning and permission gathering may go into what looks like a simple operation. Or it may be a very simple planning job - for example, a roof outside of a congested area and outside of any flight restriction zones.
No matter how complicated the planning required, a roof inspection by drone is still most likely going to be cheaper and faster than erecting scaffolding and sending people climbing!
The best bet is to contact me for a quote and I will do an initial feasibility study and determine a price.