There's koa, and there's koa...
What I mean is that plain koa need not be horribly expensive, and in fact, I've found it almost hard to give away...metaphorically speaking. Plain koa is like a very nicely enhanced mahogany, and it is very available since everyone wants the bling stuff and is willing to pay for it.
One of the luthiers' little secrets is that plain koa often sounds better than the highly figured stuff because the grain is straight and even. This means that it can actually be worked thinner to be more responsive, and the more continuous wood fibers do not damp vibration as much as curly fibers which result in tons of "run-out".
That said, mahogany is just fine, but bear in mind that it's not about a species, it's about a specific instrument. Use your ears, not your eyes if you're a serious player.
My go-to uke is an outrageously figured koa one. It sounds just fine, but we've made better sounding ukes, too. I happen to really like this one...it is an attention grabber, and having played it for four years now, I'm very used to the feel and playability, but I'm going for a spruce/milo tenor next, and then a spruce/Brazilian rosewood concert.