Road bikes are expensive. "Entry level" bikes tend to be around $600-$700, and go up from there. Certainly you can find them cheaper than that, but as far as I know, there's no such thing as a cheap road bike. I don't know why that is--my theory is that they never had the popularity boost that mountain bikes did that saturated the market and drove prices down. Dunno.
That said, you can probably find a used one for $300-$400. Also, Giant tends to make affordable bikes, as does a company called Felt.
Whatever you do, make sure to buy from a real bike shop, not a Wal-Mart. The bikes at big-box retailers are universally piles of crap, frequently assembled incorrectly and sold by someone who doesn't know a down tube from a chainstay. Go to several bike shops around, see if they have anything used, and look at their lower end bikes. And
make sure it fits. There's nothing more important, especially considering you just had surgery on your knee. An improperly fit bike can destroy a knee just as easily as running.
Things to look for? Ideally a frame made of aluminum, though at that price it may have to be steel--which could be better, as steel, despite weighing more, gives a smoother ride. Make sure it has Shimano or Campagnolo components. It's OK if the cranks and handlebars are some off brand, as they certainly will be at that price point. Just make sure the drivetrain isn't gonna come apart on you--the most important components are the front and rear derailleur. For most of the other components, you're likely to get something made by the manufacturer or an off brand, and that's OK.
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Giant OCR3 would probably be a good entry level choice for you. MSRP $600.
Let me know if there are anymore questions I can answer for you.
EDIT: You said you were planning on eventually racing it. The use of the bike is going to be a big determiner in what you buy. If you want to get somewhat serious and eventually race, make sure to get something that will put up with it, like the Giant above. If you're just looking for something to spin around the block on, it doesn't really matter what you get.