This research focuses on an application-based network resource selection algorithm in integrated LTE-WLAN systems. First, we study the structure of the LTE/WLAN overlaid systems and then propose an approach to select the network on LTE and WLAN interfaces in a user equipment. In the study we will test network's behavior change with the change of number of new arrival nodes to our model with different quality of service (QoS) and type of service (ToS) settings when implementing the network selection algorithm in OPNET. A part of this algorithm works with fuzzy logic controller block which calculates the threshold needed for comparing data usage with the remaining free data for uplink with a cost effective aim. The procedure of calculating the threshold is also explained. This network selection algorithm, gives a better result in terms of QoS for real time applications, less delay variation for cellular network for uplink data transmission and uses the pre allocated cellular data (total data amount of download plus upload) as much as it can, avoiding extra costs.