The digitalisation of the energy sector is giving rise to energy data spaces that aim to support secure, interoperable, and sovereign data sharing among stakeholders. While the focus has mainly been on technical aspects of data spaces, the economic dimensions, particularly the allocation of costs, are underexplored. This paper addresses this gap by examining principles and methods for cost allocation. We review ongoing European initiatives for energy data sharing and discuss how it can generate value while ensuring efficiency and fairness in cost allocation. We identify proportional and weighted proportional allocation rules as robust and implementable solutions. In addition, we briefly discuss governance options for fair access, data sovereignty, and economic sustainability, emphasising the complementary roles of public coordination and market mechanisms. We propose policy recommendations for a sustainable and equitable energy data ecosystem design in Europe: (i) the establishment of a single coordinating entity for a European energy data space, (ii) adoption of proportional cost allocation as default principle, (iii) distinguish between regulated and non-regulated exchanges, and (iv) incentivise early participation and data contribution.