IMO it's a reasonable idea. In campaigns that provide a plethora of magic items, why would someone spend XP to manufacture one (along with costs and time requirements) anyway? Now, in a d20 Modern campaign without any magical item except those manufactured by the players, this is of course another matter entirely. In any case, using an action point is IMO still a good idea. Note that rather than converting Action Points into XP (1 AP = 100 XP worth?), you could rule that 1 charge of a wand costs 1 AP; 1 spell level in scroll or potion costs 1 AP; etc.
ALSO: let all PCs contribute to the cost in AP. If a character must manufacture an item that is almost necessary for the adventure, all PCs should participate into the magical ritual and manufacturing (not just the mage), thus every PC contribute some action points.
In any case a good idea I will remember for my own games...