This will also help with 'On the shoulders of giants' achievement expect here you only need to focus on 1 development department.
To earn resource points complete practice programmes and achieve targets in qualifying and races. To maximise your spending it would make sense to first invest in all 5 efficiency levels of a development area prior to buying upgrades:
Chassis = Chassis Factory Efficiency 1/2/3/4/5
Aerodynamics = Wind Tunnel Efficiency 1/2/3/4/5
Powertrain = HPP Factory Efficiency 1/2/3/4/5
Each of the 5 efficiency boosts reduces the cost of upgrades in that area by 10%. So with all 5 levels in place individual upgrades will cost 50% less resource points. This should save some time (aside from the initial cost outlay on the 5 boosts).
Note there is no such saving for durability upgrades.