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There are no training costs that I'm aware of and any training requirements are at GM discretion (e.g. having to find a fighter trainer in town and pay them or simply gaining enough exp means the character inherently learns or gains any new abilities).
I personally prefer to accumulate xp from encounters and do calculations after the session has ended and let the player know if they will level up for planning. Then during the next session allow them to level up (as some of my players prefer to leave character sheets).
Retraining does have costs and I also advocate using those rules especially with novice players (I allow one retraining per session). It allows players to test different ideas without being locked if it doesn't work out well but as a GM you'd also need to be mindful not to break prerequisites in a feat chain (such as two-handed fighting feats)