I think you should plan to spend a thousand dollars or more. Just get yourself a nice bike you can use for awhile. My bike cost about 1200 and it's about a Honda Accord V-6 level bike, if you see what I mean. Not a POS econobox, and not a Porsche either.
The most important thing is fit. Make sure you get fitted. If you think the bike shop doesn't know what it's doing go to one that does. Don't buy from a big box retailer. Go where you can get personal attention. Buy as much bike as you can afford. It's nice to have quality components. They make a difference in comfort and safety and if you're on your bike more than an hour or two, it matters.
You'll also want a couple pairs of shorts with chamois inserts, a water cage or Camelbak, a tiny tool kit that fits under the seat (for flats or whatever), maybe a computer to keep track of mileage and mph and so forth. Definitely a helmet, maybe some safety goggles, and a rear view mirror for your glasses or handlebars. Get some gloves, or I just use my weight training gloves. Maybe you can wait on the special shoes and jerseys and stuff. There is lots of gear.
I ride with a local cycling group. I feel safer that way because of all the hostile drivers and rednecks on the road here in NC. People have cell phones and can help you if you have a problem. There are several rides a week to join and you make friends and so forth.
Wherever you live, there are likely to be cycling groups.
I love this new sport, I don't regret spending the money. Actually, it was an investment. Tomorrow I'm going on a 30 mile ride at 10 am. It's a great cardio, and fun.