Financial factors are among the most widely cited bottlenecks around biodiversity monitoring but are relatively poorly studied, compared to monitoring methodologies. The existing body of literature on the cost-effectiveness of monitoring focuses heavily on the hypothetical costs of generating data rather than the practical realities of undertaking and managing monitoring. To address this we used a combination of surveys and semi-structured interviews with 67 biodiversity monitoring managers to provide an in-depth exploration of 1) what are the main rivers of their costs, 2) how different factors affect their cost-effectiveness in generating biodiversity monitoring data, 3) What is the scale and economic value of volunteer labor and 4) what are the main cost bottlenecks and spending priorities. Analysis of these responses demonstrates that monitoring efforts are able to generate more data at a lower cost when they have a) higher numbers of volunteers, b) greater densities of sites and c) monitor a wider range of taxa and habitats. Volunteer labour was worth millions of Euros to these organizations, sometimes more than their total costs. Total budgets and volunteer recruitment and retention were the main cost related bottlenecks among respondents, while staff recruitment was the highest priority for new spending.  The results and discussion around them highlight the challenges faced by biodiversity monitoring organizations, particularly in recruiting and retaining qualified staff for the long-term. We produce a series of nine key messages and six recommendations for policy-markers and funders going forward.