Over the years I haven't had many major writing expenses. But what I HAVE spent my money on to further my career is ... books. Writing how-to books, research titles, and novels (this is vital to any fiction writer). These tomes have proven to be my most valuable investment. A great way to buy these books is through the Writer's Digest Book Club. I was recently surprised to realize I've been a member for almost ten years!
Second, I've been a subscriber to Writer's Digest & The Writer magazine for nearly a decade. These may not seem like huge investments, and that's what's great. They aren't.
I'd like to offer some encouragement today to those whose budgets are slim: It IS possible to succeed as a writer without going to a writer's conference (though I certainly am not debunking going when the time is right for you). But that's the key. Make sure the time is right for YOU. It's very easy to feel pressured to do things you're not ready for. Listen to your heart, and if you're a Christian writer like me, trust that the Lord will show you exactly what you're supposed to be doing (and investing in) at this point of your career. Sometimes the right thing for you is completely opposite from what everyone says.