Wildland-urban interface (WUI) homeowners who do not mitigate the wildfire risk on their properties impose a negative externality on society. To reduce the social costs of wildfire and incentivise homeowners to take action, cost sharing programs seek to reduce the barriers that impede wildfire risk mitigation. Using survey data from a WUI community in western Colorado and a two-stage decision framework, we examine residents' willingness to participate in a cost sharing program for removing vegetation on their properties and the amount they are willing to contribute to the cost of that removal. We find that different factors motivate decisions about participation and about how much to pay. Willingness to participate correlates with both financial and non-monetary considerations, including informational barriers and wildfire risk perceptions, but not with concerns about effectiveness or visual impacts. Residents of properties with higher wildfire risk levels are less likely to participate in the cost sharing than those with lower levels of wildfire risk. We find widespread, positive willingness to pay for vegetation removal, with the amount associated negatively with property size and positively with respondent income. These results can inform the development of cost sharing programs to encourage wildfire risk mitigation on private property.