Costs and strings attached
Renting a car from M-Sport for a WRC round will cost you around 150.000Euros all included, for that you get a customer car. If you want a factory spec car then you need to be someone "special" and then it will cost you over 200.000Euro. "Special someones" in the past have included all Jouhki drivers, Wilslow, Tuohino, duval when privately entered.
The difference between the factory specification car and the customer car can vary depending on a number of factors. Top spec cars will have all the titanium parts which can easily add up to being 100kg lighter than customer spec which use steel. We are mainly talking about suspension parts, cross members etc.. Also thinner windscreens and aluminum bonnets will make a difference. people who doubt this should check the difference in weights between the 2 during scrutineering when the cars are empty. a difference of 100kg can have an impact of 1 sec/km on average.
Next thing is power. Top cars have in reality around 360 HP and customer cars will lack anything up to 40HP. It is very hard to see or hear the difference in power on turbochaged cars because the initial kick is always there. The difference is at the high end of the RPM's when some will run out of breath as they get to the REV limiter and others keep going to REV limiter much faster.
After this comes shock absorbers and setups but its another long story....
Hope this gives a little insight to the very intricate world of WRC.