You can pick up 80% frames, but the availability is spotty at best. Slides are available from various sources online from popular dealers. 80% frames run from around $75 to $150+
Completed frames are available too, mostly from popular dealers. These can go from $150 to $400 for standard frames, $400 to $600+ for STI frames, and up to $2000+ for titanium pieces. On average, plan on spending around $250 for a decent frame like a Les Baer. Titanium frames can be ordered, but are custom one-off jobs by specialty builders. They usually EDM these frames and you will pay through the teeth for one, but MAN THEY ARE COOL!!!!
The 1911 can be thrown together from a box of parts and will generally function fine (similar to an AK). The difference is taking the time to fit the following items:
barrel bushing to slide
barrel to barrel bushing
barrel locking lugs to slide
barrel pivot lug to frame (and slide)
barrel feed ramp to frame
slide to frame
sear to hammer
trigger bow to frame
grip safety to trigger
When these items are properly fitted, the pistol jumps up considerably in accuracy and reliability.
The important things to watch are sear/hammer fit and spring tension. If this is off, the pistol will have a crappy trigger feel and could be dangerous. Slide to frame fit is important for accuracy and functioning. Barrel to frame and slide is for consistency and accuracy. Oh yeah, the disconnector better function properly or you could go full auto.
If you're looking to build a 1911 I HIGHLY recommend getting a copy of "The Colt .45 Automatic - A Shop Manual - Volume 1" by Jerry Kuhnhausen before buying any parts. It is probably the best single book ever written for the 1911 enthusiast. Even if you aren't building a 1911, this book can help you diagnose problems with your current pistol and fix them properly.
Some of the drop-in parts are pretty close. There are drop-in hammer/sear sets that will give a reasonable factory trigger feel.