Well, it doesn't have to be. I've been riding TT's for three or four years and until this year the only thing I bought was a pair of aero bars - which you've already got.
I ride my normal road bike, normal clothes and helmet. I got a light hearted telling off last year for leaving my water bottle in the cage - "you'll knock loads off, leave it at the start".
The one thing that's stopped me spending is that as I'm only chasing my own times it feels like I'm cheating if I get a better time because of the gear. At the moment I know that the improvements in my times have come from losing weight and working harder.
BUT........there are worse things you could spend your money on!
It is addictive, I must admit that when I'm riding I think "what the hell am I doing this to myself for!" but as soon as I've finished I'm looking forward to the following week. I've never looked forward to Wednesday nights so much - bring on the new season