In this paper we describe the development of an online tool for assessing the foundational knowledge and competencies essential to understanding money and building financial literacy. We present findings from an initial study using the tool to (i) measure basic skills with money among secondary students with intellectual disability (N = 85) and (ii) test predictions regarding item difficulty. Item difficulty was explained using a system of 36 levels of complexity based on task type and variations in the number and combination of denominations presented. Implications of the results are discussed in terms of how sensitive assessments can inform practitioners about how they might adapt tasks to differentiate their teaching.