"El Cheapo" Road Bikes

The road bikes that I have now (and the road bikes that I have owned before) are not cheap. But while every one of them is worth over a thousand dollars, I'd like to believe that they're worth every penny, and they would have cost a lot more if I were to buy a brand-name bike. That said, I had been paying attention to the prices of the components for awhile now, and I do have  a better idea of how much things cost compared to most people. Here's a break down of the costs of a full carbon "example bike" that I would build for under $1800. It would be similar to the current bike that I ride.

Component Cost (USD) Vendor Comments
Carbon frameset $480 eBay This is how much I paid for my new frame in 2014. It included the frame, the fork, seatpost, seatpost clamp and the headset bearings. There are many different types to choose from on online sources, but not every listing come with all the parts that I listed, so pay attention or you might end up needing to buy extra parts. Typically you get to choose from 12k/3k, matte or shiny finishes, and BB30 or BSA bottom brackets.
Carbon wheelset $430 eBay Again, there are many online vendors with full carbon wheelsets, with different rim depth to choose from. The 50-60mm rims look awesome, but if you're a climber, go with some shallower and slightly lighter. Like the frames, the wheelset can be 12k, 3k or UD finish, matte or shiny and with campy- or Shimano-compatible hubs. If you don't care for carbon wheels, there are ~$150 alloy wheelsets online as well; it'll save you a lot of money.
Brakeset $60 eBay You can find lots of Tektro brakes on eBay. Their R580 model is at about this price range. They're not quite as good as the Shimano, but they'll do just fine.
"Groupset" $200 eBay If you really want an inexpensive, reliable and high-performance groupset, a little-known Taiwanese company Microshift is a very good alternative. So good that I've been using it on all my high-end bikes. Their Centos groupset is lighter than Shimano Ultegra and a lot cheaper. At $200 you can get the integrated 10-speed "brifters" plus the front and rear derailleurs.
Crankset + Bottom Bracket $150 eBay Look for new-old-stock models. I bought my Raceface Cadence crankset along with the bottom bracket for under $100. That's harder to find nowadays, but there are also a lot of other options from FSA, Truvativ and Shimano.
Chain $30 eBay 10-speed chains are surprisingly inexpensive...
Cassette $50 eBay There are lots to choose from. Shimano 105 and SRAM PC-971 cassettes are probably the safest bets in terms of getting the most out of your money.
Alloy stem $20 MEC You can get a comparable stem on eBay for a few dollars cheaper, but you have to wait for shipping.
Handlebar $50 eBay highly recommend the FSA Omega Compact handlebar. It's not the lightest, but it's also not terribly expensive, and it is very comfortable regardless of your riding position. There are cheaper alloy ones for less than $30 if they work for you.
Handlebar tape $4 eBay If you can wait, they go from dirt cheap to very expensive online. If you can't wait, your local bike shop will probably have something starting at about $10.
Rim tape $3 MEC  Can't do without them.
Tires $30 Nashbar The Nashbar Prima 2 tires are awesome. They're light, they're cheap, and they're durable. Look for when Nashbar has a sale. MEC also has a lot of competitive products.
Saddle $30 MEC or Nashbar I quoted a very low number, because the saddle that fit me best was a dirt-cheap saddle from Nashbar that I bought on sale. But everyone's butt is different, so don't be surprise if you have to spend a lot more money on it.
Carbon stem spacers $3 eBay You may not even need them at all. I only use two tiny ones myself.
Cables and housing $10 eBay Without the brake cables, you'll never stop your bike. There are "Jagwire" cable and housing packages available on eBay. They used to be more expensive, but the cost has gone down significantly in the last year.

You can add up the numbers from there. If you're patient, sometimes you can get the parts on discount. That said, if waiting and putting parts together aren't your cup of tea, then this put-my-own-bike-together exercise isn't a lot of fun.