you can usually build your dream AR for a lot less than you can commonly buy it for, however you're going to have to invest in some tools to make it feasible and relatively easy. its worth it if you plan to build more than one, but depending on what you may have around the house already, the cost of the tools may offset the difference between building and having someone build for you.
plan on spending around $100-$150 minimum on the speciality tools required for the assembly. you'll spend a bit more if you get some of the luxury tools to make the process simpler. and thats assuming you already have some basic tools around the house already.
things you'll need to consider tool wise (in no specific order)
this is by no means a complete list, but should cover most of the basics needed
[*]armorer's wrench (for barrel nut, flash hider and buffer tube nut)[*]gas tube alignment tool for getting the barrel nut indexed properly (make life easier item)[*]upper receiver vice block[*]lower receiver vice block[*]torque wrench[*]roll pin starter punches (Make life easier item)[*]trigger guard roll pin tool [*]standard pin punch set[*]gas block/front sight base bench block (make life easier item for driving that pesky gas tube roll pin)[*]some type of bench vise with at least a 4" jaw opening[*]a non-marring hammer for driving pins - i use a double sided rubber/plastic head hammer[/list]
some barrel makers will offer to headspace a bolt to the barrel they're selling you if you buy the bolt from them too (white oak will do this for sure). otherwise a set of gauges might be on the list too. call around, you may be able to pay a small fee to a LGS to have them check the HS on a barrel/bolt for you.
one other big thing to consider. be conscious of shipping costs and try to bundle your purchases together if you're buying online wherever practical. sometimes saving $10 on a part's list price by using a separate vendor can be EASILY eaten up with the freight costs of ordering from too many vendors. always remember to take the total part+shipping cost into consideration when putting your shopping list togther.
HTH