OSPF - Cost of summarized routes
According to the official RFC of OSPFv2 (see link below), it states that when routes are summarized, the cost is equal to the largest cost of any of the component networks. However, in practice, most OSPF implementations, including Cisco’s, use the cost of the subnet with the lowest cost that falls within the summary range when calculating the cost for an OSPF summary route. This is done to ensure that traffic is routed through the most efficient path.
So, while the RFC does state one thing, the actual implementation can vary depending on the vendor.