If $190 counts as affordable, see if the
Cypress CY4500 EZ-PD Protocol Analyzer will do what you need. Some of the features:
- decoding USB Power Delivery packets in real time on the CC lines
- Decodes PD 2.0, PD 3.0, Extended messages and USB4 messages
- CC line Voltages, VBUS Voltage and Current monitoring
- Graphical view of the CC lines, VBUS Voltage and Current
Only monitors the PD protocol - it can't modify or spoof the protocol.
There's also the
Google Twinkie project and various forks/improvements (such as
Twonkie).
features:
- Sniffing USB Power Delivery traffic on both Control Channel lines (CC1/CC2)
- Transparent interposer on a USB Type-C connection
- Monitoring VBUS and VCONN voltages and currents (Twonky supports USB PD 3.1 EPR voltages up to 48V, Google's Twinkie only SPR voltages up to 20V)
- Injecting PD packets on CC1 or CC2
- Putting Rd/Rp/Ra resistors on CC1 or CC2
Google the terms to find out more.
There's someone selling assembled Twonkie devices for 112 EUR here:
https://shop.3mdeb.com/shop/open-source-hardware/twonkie-usb-c-sniffer/Disclaimer: I have used exactly none of these devices