High training makes a unit cost more, making them more difficult of access to people. They will also not improve much over the course of battles.
If you aren't going to fight any battles, than yea, training low-training men yourself will cost you a lot more than saving up and buying the fully trained unit later, in training costs and especially in pay. However, if you are going to have many battles, spending on high-equipment and low-training unit should grant you a pretty similar CS/gold ratio, but it'll improve for free over the course of the coming battles.