I found this recently, and started experimenting with it for my fire truck, as I'm not happy with what I have at the moment.
This is a DIY solution by "TheDIYGuy999" that has been developed far enough to compete with the expensive ones. It can be put together for around 20-30USD and features multiple simultaneous sounds (engine+turbo, air brake, horn, siren, indicator, reverse beep). It can control an ESC to simulate mass and inertia, and integrates with the Tamiya 3-speed gearbox. It also supports a vibration unit and lights.
The best thing is of course that it's open source, so it's infinitely customizable. It runs on an ESP32 dual core microcontroller and can be programmed in the Arduino environment.
Source files and instructions:
https://github.com/TheDIYGuy999/Rc_Engine_Sound_ESP32
Videos:
https://www.youtube.com/user/TheDIYGuy999