Look for a used bike on CL. Plenty of very high quality bikes (Ultegra quality) for less than $400, and no tax. Unless I'm attacking hills, there isn't that much gain in speed between a good 23 lbs steel bike and a light 17 lbs aluminum. My workhose bike is a 24 lbs (with Brooks saddle) Cr-Mo steel road bike with 28/32 tires.
You may gain 1-1.5 mph in average speed with a light road bike. That's noise in my book. Do you cruise around 17mph on flat land with no wind? If not, then you still need to train the "motor" before spending more $ on a lighter bike.