Most cost effective source for residential wire and conduit?

Just some comments. If you are going with direct burial UF is the minimum you should use. Unless you are going to drive another ground rod, 3/C is probably going to be required. 10/3 UF-B Wire w/ Ground

Agreed that single conductor THHN is the way to go if you stick with conduit. 10 AWG THHN/THWN-2 Solid Building Wire - sold by the spool has a 500' spool for $200.

500' is a looong way for 120 VAC. If you use one of the voltage drop calculators, a 10% voltage drop (which is the maximum usually tolerated, 3% is more common), 10AWG is only good for about 8 amperes. Are you sure you want to do this?