I am new to ar15's and thinking of building my first one. It looks easy enough to assemble a lower receiver. But what about the upper? How much more difficult is it to assemble than the lower? And how much do the tools cost to build an upper receiver? Thanks for any information.

uppers arent at all difficult... you could acquire all the tools for under $100...

the only specialized tools you would need are a barrel wrench and action block (and a torque wrench, depending on who you ask)... barrel wrench can be had from brownells for around $30, and the action block is about the same... thats about it... you would also need some grease for the barrel nut (again, depending on who you ask)... you will also need a punch for the roll pin that secures the forward assist in place... thats about it...

here under books, look up the tm for the m16/m4...  it explains in detail the proceedure for assembling and  barreling an upper...  you could probably do the whole thing in 30 minutes...

great thing about buying your own tools is after the initial investment, you can build as many rifles as you want!  (and believe me, you WILL build more!)