Bulbs need replacing every 6 to 12 months. Ballasts, as long as they're not overheated, should last years. Even if the ballasts do need replacing, it's not terribly difficult.
I bought a used TEK brand 8x54w T5 for my Peninsula. The used fixture with one bad ballast was $80. Polished up the reflectors, bought two new 4x54w ballasts, $45 each, wired them up, 8 new ATI bulbs, and I got a like new fixture to light my 140g system for under $300. I'm as happy as I can be with it.
I've never priced a dimmable T5 ballast... I'd assume they'd be a bit more money, but probably not that much more.
Even the cooling fans wouldn't be that big a deal to replace... if you needed to.