It seems that it's the norm these days to select the low bidder for contracts. I used to administer the bidding process for good sized engineering and construction projects relating to environmental cleanups. In my experience, you usually don't want the low bidder, especially if the bid is lower by a large margin. It typically means they do not understand the scope of work, will not provide satisfactory results, will not complete the project on time, or all of the above. As a matter of course, I would toss out both the high and low bids and select one in the middle.
It's a sad statement that substandard workmanship is winning out over quality based on money alone. I think we all know that once a landscape is let to go to hell, the cost of restoring it is far more than it would have been to maintain it properly in the first place. The old adage of penny wise and pound foolish comes to mind.