In Europe there is substantial variation in the costs associated with applying for citizenship both between countries and over time. Yet how the costs of applying for citizenship affect naturalisation rates among immigrants remains remarkably understudied. In the Netherlands, application fees increased markedly in recent years from 336 euro in 2003 to 901 euro in 2020 for a single application, with significant hikes in the fee in 2010 and 2011. While simultaneous policy changes in civic integration requirements for long-term residence obfuscated the effects of higher fees and likely resulted in higher naturalisation rates among non-EU immigrants, we expect that for EU immigrants increased fees directly affected the cost-benefit calculation of applying for citizenship. Hence, in this chapter, we analyse naturalisation rates among EU immigrants in the context of increasing application fees by using longitudinal microdata from administrative registers on the complete immigrant population between 2007 and 2014. We use a two-step identification strategy. First, we apply a single-difference regression, based on a fixed-effects model, to investigate immigrant naturalisation rates in conjunction with increased application costs. We subsequently explore impact heterogeneity by household income and use a double-difference regression, based on a difference-in-differences model, to test whether the relevance of the fee increase is conditioned by income groups. Results show that increased fees are associated with an overall declining trend in naturalisation rates, especially for immigrants from low-income households. The impact of the fee increase for low-income migrants is comparable in magnitude to the relevance of having a low rather than high level of education for the propensity to naturalise. These results contribute to existing research on economic requirements for immigrant naturalisation and highlight the disproportionate consequences of high application fees.