Hi, i'm not sure what you consider to be a high price for the code and was leaving this for other experts, as you have not yet had a response I will see if I can help you.
I use a company in Anglia for all my vehicle related codes, when I checked your stereo it was £22 for the code, let me know if this price is acceptable?
These places usually buy database access to or from, the vehicle manufactures, original records and need to provide security confirmation also.
Your could have the code found by de soldering and reading the contents of the eprom to a hexadecimal 'dump' using fairly cheap and standard chip programmer/readers and manually decoding it, not sure if this is your sort of thing but it works for me.
Some diagnostic units can pull the code via the obd port over the vehicles bus, I use Opcom to pull security pin numbers and program in keys, etc, but I haven't yet tried for a stereo code.
Both of those options are over the cost and a lot more work then for £22 for the code provided to you quickly and easily.
There isn't a simple way to decode it from the serial number that I am aware of I am afraid yet, it's memory dump, original database or pulled out with a computer unfortunately.