Your RCD300, with the code, is worth £10-£30 (if you can find anyone to buy it, try ebay) and standard RCD510's are around £170-£200 with the RCD510 DAB's are another £30-50 on top.
Canbus Gateway can be bought from Eddie-NL for £70 (IIRC) or he supplies and fits for around £100 although you'd need to ask Eddie about the correct price.