I've begun holding onto the amulets I win in quests, in preparation of the time when they become the cost effective stat booster however I'm not sure if they're worth it given theres a chance you don't get the tome from them, and instead wonder if I should sell them off and save the sP for other (most expensive) boosters with better/definite chances of increasing stats.

What do you think?

On average, you can expect 1 tome for every 6 amulets you turn in. I save mage amulets and turn in a large number at once. Each time I've done that, the average matches what others have reported - roughly 1 out of 6.

It's worth it to keep the amulets and turn them in for tomes if your intention is to acquire stat boosters.

Good to know! I'll continue hoarding them then.