My son, my 3 younger brothers, what DH has told me about himself, just about every boy I've ever known...doesn't NEED the sort of official dressup stuff that girls do (or moms think girls do). My son could put on a hat, put a belt diagonal across his chest, and he WAS a pirate. Period, no question, what do you mean do I need a costume? He was a pirate.
That makes many/most boys soooooo easy. Ask if he wants to dress up, ask what he needs to dress up if he wants to do so, see what he says. Maybe he'll be one of the easy boys who doesn't require large amounts of money to be spent to let him feel like another person/character. Cross fingers!

DS's Indiana Jones outfit originally was a pair of khakis and a buttondown shirt. That's IT. Later he found a whip in the toy section, then later again he found a fedora at WDW, and even later we found a cheap pleather coat. But even with the khakis and buttondown shirt, you couldn't have convinced him he didn't look exactly like Indy.
Lastly...as a sister to 3 younger brothers, if you're going to do a princess and prince, make them UNmatching prince and princess. So...think Eric and Aurora or Philip and Ariel. etc.