Sniffing your own packets isn’t illegal; nor is giving incorrect responses or sending invalid packets.
Invalid packets, such as those with an IP address you don’t control, should be dropped by your ISP. If the packets cause errors or bring down the ISP’s systems, that’s the ISP’s fault for using such brittle software.
You don’t even need to spend $20,000 for a packet sniffer… you can download a free one for Linux. If you really want to know exactly what you’re sending and receiving, it’s best to use a separate machine anyway (placed in between your main computer and your router or modem).