There are no rules for recharging.
If you want a house rule, the simplest way to do it is to pretend you're crafting the original item over again, but pro-rate by the number of charges you're replacing.
For instance:
A wand of Bull's Strength costs 4500 gp, for 50 charges. That works out to 90 gp per charge. If a PC expended 20 charges and want to replace them, you might consider that to cost (90*20)=1800 gp. Then you'd charge the player for gp, XP, and time as if he were crafting a new wand with a market price of 1800 gp. (He'd also need the Craft Wand feat and a way to cast the appropriate spell.)
If you want recharging to be harder, you might increase the cost-per-charge by 50% for recharging purposes. I wouldn't recommend lowering the price though, because then it becomes more attractive to carry around many, many wands.
Of course, even if you allow something like this, it's impossible to recharge a fully expended item (like a used scroll or a wand with 0 charges).