Have run marathons before.
+1 to all comments valuing shoes. To cut the cost, research your local running stores, learn their sales or other savings programs. With planning, you can get a discount on your inevitable next pair of shoes even if it's too late for this training season. (Do the stores give discounts for turning in old shoes to recycle? By using coupons? By having clearance sales or 2 for 1 discounts when the manufacturers bring out new models of their shoes?) If you're going to keep running, you lose nothing by purchasing a new pair when they're cheap. You can even break them in gradually and preserve your old pair by alternating workouts. Also, if you are getting a discount by using an "x % off" sale, buy your Gu at the same time to maximize the discount.
+1 to signing up early for your race(s). Early is always cheaper, later is always more expensive.
To maximize the ability to finish on marathon day, I strongly support running a couple of practice races, exactly as you are planning. The experience you get ensures your success on the big day, so in a sense the small investment in race fees for the shorter races is protecting the bigger financial expense of the marathon itself. It's still a matter of paying for an experience, but I think it's well worth it. I love being part of a running club sometimes, participating in races, etc so agree with posters who feel these can be experiences worth the money.
Fwiw, check also into the cost of running clubs and such. Obviously if you prefer the solitary nature of a training run, this may not be what you want, but it's amazing how uplifting a supportive group of fellow runners can be. You get a whole new set of friends in the same time you're already putting into your training program. The financial aspect here is that some running groups are expensive for-profit activities while others are dirt-cheap nonprofit clubs. If your city has a good cheap running club (free to $60/year range, for example) you might even come out ahead financially through training tips, referrals to shoe discounts, and so on.