@P_enta is very good at getting most of the car modeled, which does take hours of his time, and also gets the other parts co-ordinated so that the mod is actually made. Without his help, someone else (usually the commissioner) would have to spend time modeling it (takes longer and also, who knows what quality it will have?) and they'd have to just ask around to get people to contribute to getting it in game, without the notoriety of P_enta. Additionally, he documents the creation for the car on his Youtube page so that the commissioner can see what he does. All of these things make a paid product.