First: You only get to improve your score once through mundane training. Period. If you can find a Manual of Improvement, or a magic throne that improves your ability, or a ring of wishes, that’s different. But vigorous weight training is going to build your muscle strength one time only, although it might grant superficial improvements, like better-looking muscles.
The cost per month is 400 GP for equipment, training texts, health supplements, and the like. At the end of every month, roll 3d6: if this is higher than your current score, that’s it, you’re done; your new score is the result you rolled. So: training might get you an enormous jump in ability, or it might just get you a single point.
If the die roll isn't better than your current score, you can keep training. You can also study under a teacher; teachers cost extra (whatever the teacher or the reaction roll decides,) but you roll 4d6 instead of 3d6 … assuming the teacher isn’t a fraud. Rolls above 18 raise your score to 18 max.
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