Every so often the library is asked to confirm or locate a report attributed to NAR that says graffiti lowers property values by 15 to 20 percent. Unfortunately this report does not exist, at least to the best of our knowledge as well as that of our Research group. While NAR did sponsor an anti-vandalism campaign in the 1970s, there was no research attached to it. However, there are a few studies available that discuss graffiti and vandalism in the wider context of the economic impact of urban property crime:
Households experience disutility from crime near their homes and as a result would be expected to reduce their bid prices for housing in areas where crime is more prevalent. This theoretical prediction is largely borne out in the existing literature on crime and property values, although the degree to which higher crime affects property values is a matter of debate. Much of the difference in estimates likely depends on estimation techniques, the measurement of crime as well as property values, and the size of the sample studied.
Crime & residential choice: a neighborhood level analysis of the impact of crime on housing prices. Journal of Quantitative Criminology, Dec 2006.
Our results indicate that the average impacts of crime rates on house prices are misleading. We find that crime is capitalized at different rates for poor, middle class and wealthy neighborhoods and that violent crime imparts the greatest cost.
On Google Scholar you can find a few more articles, though several of them are UK-based research. Of course all bets are off if that graffiti turns out to be a Banksy original. Those can go for well into the six figures.