<p>Incorporation of micron-to-nanometer fillers into fiber-reinforced epoxy composites is a viable way to improve mechanical performance. The study presents an in-depth evaluation of micro- and nano-scale copper additives within GFRE (glass fiber-reinforced epoxy) composites subjected to similar fabrication and loading conditions, accompanied by a performance-cost analysis adopting a multi-criteria decision-making (Entropy–TOPSIS) technique. GFRE laminates were manufactured incorporating three different contents of 0.2 wt%, 2 wt%, and 4 wt% micro/nano Cu fillers utilizing controlled sonication and mechanical stirring. The study simultaneously evaluates the tensile characteristics (on- and off-axis), hardness, void content, and cost-effectiveness. The results, presented as mean values with related standard deviations shown in the figures, indicated an improvement in on-axis tensile strength of 21% and 20.6% for 4 wt% micro-Cu and 2 wt% nano-Cu, respectively. However, improvement in off-axis tensile strength of 33.8% and 38.2% for 2 wt% micro- Cu and 2 wt% nano-Cu. Maximum enhancement of on axis strain is 65.7% for 2 wt% nano-Cu and for off axis strain of 43.6% is attained also with 2 wt% nano-Cu. The analysis indicates nano-Cu performed better at modest contents, while micro-Cu is cost-effective and more stable at higher contents. These outcomes serve to optimize GFRE composites for structural applications that require performance-to-cost balances.</p>