Ever-increasing bandwidth demands and higher flexibility are the main challenges for the next generation optical core networks. A new trend in order to address these challenges is to consider the impairments of the lightpaths during the design of optical networks. In our work, we focus on translucent optical networks, where some lightpaths are routed transparently, whereas others go through a number of regenerators. We present a cost analysis of design strategies, which are based either on an exact Quality of Transmission (QoT) validation or on a relaxed one and attempt to reduce the amount of regenerators used. In the exact design strategy, regenerators are required if the QoT of a candidate lightpath is below a predefined threshold, assuming empty network conditions. In the relaxed strategy, this predefined threshold is lower, while it is assumed that the network is fully loaded. We evaluate techno-economically the suggested design solutions and also show that adding more flexibility to the optical nodes has a large impact to the total infrastructure cost.