As mentioned, this depends on the choice of mana potion, small mana potions will always return more mana per gold coin, but you are hindered by their lower gain meaning longer time to fill and deplete your mana bar.
In relation to training wands, this can largely depend on the price of tibia coins on your server of choice.
Assuming the 500 charges relates to 500 ultimate mana potions, if the cost of the training wand is less than the cost of 500 mana potions then the wand is the better option. However, do keep in mind the 10% boost from using a trainer bought from the store, this would mean that it would be of equal value to 550 ultimate mana potions.
Then of course you have to factor in your time and effort, if the wand/rod is cheaper by all means go for it. However, if buying the 500 ultimates is cheaper you have to ask yourself is the difference worth the effort of drinking them and casting spells.
From my point of view, anything around 10% increase in price is not worth the time to drink potions, i would sooner buy a training item and go make food, read a book, reserach some lore etc..
Excusing the length of my post, the final point to consider is how you would spend the mana from potions, training items will return nothing but magic level, however drinking potions and making avalanche/sudden death/great fireball runes would either allow you to use them on a hunt, saving you having to buy them, or you could sell them to recoup some costs.
As you can see many factors are in play here, but ideally start with the cost of mana potions vs training items and go from there!