Depends on what your local Gunshops charge for AR15s and whether or not you are willing to order online for a savings.
You can build an AR easily for $450 JT Dist Kit and $150 average price of a stripped lower after initial cost, shipping and FFL fees. So figure around 6 or a bit less. If you can get a brand spanking new AR for that amount or close, then by all means puchase a factory one with a warranty. In respect to warranty, if you order a kit and something is wrong folks like JT will rectify the problem.
Another option to look at is how many aftermarket parts will you want to add? If you buy a factory will you have those extra parts that will be hanging around such as grips, triggers, handguards, etc..
If you have the basics of how an AR15 works and how it is assembled, then I say build. If you are pretty much totally new to ARs then I suggest you purchase new.
Good Shooting
RED