Most cost effective source for residential wire and conduit?

You need what is called "2 conductor plus ground." You might check it might be cheaper to run "direct burial" rather than conduit. Make sure that whatever you get is rated to run in conduit. "Romex" is normally "not." You can run THH style single conductors in conduit. To meet code you need a grounding conductor, and I don't think you can run bare in conduit, so a black, a white, and a green

Voltage drop depends on AMPERAGE and feet. So you need to find out what the maximum load is you will have and you may want to "scale that back" some. There are charts out there (look for one for AC/ home power rather than 12V etc) for wire gauge and "run" length