If I were you, I would jump on that. Not only is there good money in fert, but there's a real window of opportunity for you in upscale selling...dethatching, aerating, overseeding, Spring & Fall clean-ups, bed maintenance, prunning, mulching, even mowing, etc.
As for how much they're worth, I would have no probelm offering 50%. Maybe more after I had a chance to look at some/all of the properties.
Go for it...good luck!