I want to run 180' of cable to from my house to a 6 circuit main lug panel in my kennel (50 amp service). I was told that I would need 6-3 UF to do the job. However, it's going for $3.50 a foot. Does anyone have an alternative cable to suggest? Perhaps some kind of aluminum wire?

Look into using #2 - 4 wire aluminum URD cable, it is rated for direct burial.

Well.. a bit of bad news for you. At that distance you will need #4 wire for the current carrying conductors. That's three... and a forth wire for the ground, which you MUST pull. That could be a #6.. possibly a #8. And you will need a grounding electrode at the kennel as well. What I would do is bury pvc conduit and pull the wire (copper;stranded)