3%-4% of the home's value.
It's impossible to quantify--unless you could have comps--similar houses, one with good landscaping, one without. But, just thinking about houses in my area, I'd expect a differential of 3%-4%.
On top of that, the one with better landscaping is likely to sell faster. Some folks will be so turned off by poor curb appeal that they won't even venture into the home. Or, as HomeBuyer suggests, poor curb appeal is likely to suggest that there are other things wrong with the home.
And that's likely the case. Real estate investors often "Drive for Dollars"--driving around neighborhoods looking for properties whose owners are in distress. One of the obvious signals is a poorly maintained lawn and bushes.
Hope that helps.