You might not save a dime. Essentially, you should - the cost of labor, which comes out to about $100-150. What many don't consider is the skill involved in all the little things - like installing the lower trigger roll pin without breaking off the mounting ear, or making sure the upper is squared, true, and properly tightened by the numbers (three times) with an appropriate lube.
All that is in most of the assembly stickies. If you are mechanical, and have worked on automobiles, it will be relatively easier than rebuilding a carburetor or alternator.
But the real hazard is that buying the components separately means you get to pick and choose, and that's like passing a whorehouse on your wedding day. Will you stick to your commitment or stray? Things could get really expensive.
If you choose to build, figure out what kind of carbine you want, what items, what budget, etc. Then you have a clear cut goal. Otherwise, you get trapped wrangling decisions over stuff that may be entirely incompatible - like a 24" barrel with four rail handguard on a issue M4 stock. Don't build a hybrid, it winds up not doing anything well.