I have spent ages looking for cheap airport parking and the cheapest for 4 days was £35!
Take a look at uk.parkatmyhouse.com basically, people rent out their parking spaces in their drives, my mother uses it when she flies form Gatwick airport and it costs her 25 pound per week, and they have a searchable database for the whole of the UK.
Here you go first search:
5 Minutes From Robin Hood Doncaster Airport, Doncaster, DN9
This owner has a reliability rating of 98%
Secure Parking Beyond Electric Gates, only 5 Minutes From Robin Hood Doncaster Airport, Offering Park and Airport Transfer
£3.00 per day