@Gary Ennis, I'm going by memory here, but I think we furnished our 3/2 ranch with finished basement for about 10k. I think it partly depends on your price point and style of rental, as in, is this a fishing cabin or a luxury retreat? Ours was a house in the White Mountains of New Hampshire that appealed mainly to families and outdoors enthusiasts, so we were somewhere in between.
I think the approach is similar to a flip in that you decide where to spend the dough and where to save. You should have comfortable mattresses but the frames/headboards don't have to be expensive; sheets should be of good quality, as guests will likely compare this to a stay at a hotel. You want couches that will withstand the rigors of a rental while understanding that, yes, it's a rental. We also bought a pool table, ping-pong table, grill and patio furniture, and washer/dryer for our rental along with a couple of TVs, a DVD player, etc.
One area not to skimp: chairs. As was pointed out in Christine Hrib Karpinski's excellent how-to book, you don't want your guests falling on their backsides (and keep in mind that some of them may have substantial backsides, so plan accordingly!). So make sure your dining room chairs and patio furniture are decent. Like, can they hold up a 300-pound person, just in case?
Otherwise, we found a really large and sturdy kitchen table for $100 at an auction; same with end tables, lamps, etc., which we bought for something like $10 each. We found some other furnishings on Craigslist. We found plates and glasses, etc also at an auction, and would flesh them out from Christmas Tree Shops (that's where we'd replenish wine glasses, silverware, coffee mugs - can't beat a buck apiece).
Bottom line is that there's a lot to purchase when you're starting from scratch. I'm interested to hear what others have to say.