The throughput is the key indicator when sizing the right hardware. If it changes significantly, the platform and associated cost can change quite a bit.
For 1 Gbps of throughput under various use cases, you would generally be steered towards one of the new Firepower 2110 appliances.
List price is on the order of $US11k with licensing (IPS, URL Filtering and Malware protection) and support (assuming all features are licensed) adding another $6k per year.
