I tend to prefer high-magic fantasy games, where magical items can be purchased (at around the DMG prices, in D&D) in many large(ish) towns - it's common for the very well-off to have magical items. Most of these are trivial - clothes that resist tears and stains, buckets that can hold slightly more water than they should be able to, tools that stay sharper, etc.
I also extend this to magical arms and armor, sometimes in unusual ways. A villain's +3 greatsword, for example, might make a sound similar to a terrified scream when swung, or a cleric's +2 breastplate may glow with his god's holy symbol when he's struck. These effects are typically flavor, without any effect on game mechanics (though I will allow it in limited circumstances - the sword swinging may alert distant guards, or the holy symbol's appearance may grant those who see it a Knowledge(regligion) check to recognize the cleric's allegiance).
Some shopkeepers will have these applied as well, at character's request or on their own behalf (one shopkeeper actually got back at a dwarven PC for an insult in this way - he indicated that the +1 Dwarven Waraxe had a 'really useful' added property, which he upcharged for, activated by a command word...it turned it into a stepstool when you activated it).
What sort of effects would be appropriate for this type of 'enhancement'? What should I allow at no cost, and what should I up the gp value for?
How can a
clevermunchkiny player turn this against me to gain an advantage?