For sure not cheap, but if you're going all the way to Brisbane to see your son, and you're both big Springsteen fans, and the two events clash with a chance to see him in a smallish venue, you'd be daft IMHO not to just stick on your credit card and forget about it.
Why not get your son to register instead, if (as I'm assuming from your post) he's already living in Brisbane. Would make the whole process a whole lot easier, as regards mailing of tickets, currency conversions etc. Or is it a surprise pressie for him?
I saw Oasis at the Brisbane Entertainment Centre, same venue as Springsteen is playing, a few years back, 2009 maybe. Venue is about the same size as the Brighton Centre (5000-ish capacity) and is reached by suburban train to Boondall, from where it is a funky sort of nature trail walk through...