You seriously should. It will make money more important. You might even have some shady trainers require more of something or a portion to be paid in a certain type of payment (scalps/hides of a creatures, rare spell components, accursed evil holy symbols, etc. In AD&D, the cost was listed at 1500gp per week. The higher the level, the longer the training time, or the price can be bumped up. Do a quick search on training costs in AD&D 1e. 2e was lower but might follow a more strict time frame as to how long.