Abstract Background Evidence to support the use of many retention strategies in clinical trials is lacking. Despite this, trial teams still need to have some form of retention strategy in their trials to try and avoid high attrition rates. This study aimed to estimate how much this lack of retention evidence might be costing trials in Ireland and the UK. Methods We selected the top ten most routinely used retention strategies by Clinical Trial Units in the UK and made assumptions as to how each of these strategies was most likely to be implemented and the costs involved in doing this. We applied our costing model to a hypothetical trial scenario in both Ireland and the UK as well as to three published trial protocols. We developed the costing model and calculated the costs in Microsoft Excel. Results Retention strategies were often poorly specified, meaning we had to make assumptions about implementation and in some cases about the strategy itself. Based on our assumptions, some retention strategies can be extremely expensive; some of the costliest strategies included “data collection scheduled with routine care” (€900–€32,503.25), “a timeline of participant visits for sites”—with integrated participant reminder (€304.74–€14,803.70), and “routine site visits by CTU staff” and “investigator meetings face to face”, both costing (€777.67–€14,753.48). Others such as “telephone reminders for questionnaire response” (€34.58–€568.62), “a timeline of participant visits for sites”—site reminder alone (€79.18–€112.23), and “targeted recruitment of sites/GPs” (€30–€1620) were less costly compared to the other strategies. Discussion The resources invested in the use of some retention strategies may outweigh known or imagined benefits on retention. Where benefits are currently unknown, evaluation should be a priority. Conclusion More evaluation of the effectiveness and cost of trial retention strategies is needed to avoid widespread use of strategies that are both expensive and ineffective.