It is no longer possible to replace your M9 sensor if it shows signs of the well-known corrosion issue. Instead, Leica is offering to upgrade to later models. The trade-in values are about right if you take into account the most recent cost of sensor replacement.
According to the Leica website, the production of CCD sensors for the M9 Leica M9 models has been discontinued:
…subsequent deliveries are no longer possible and defective sensors cannot be replaced. Until further notice, all other repairs on the M9 model generation can still be carried out. In place of the sensor exchange, we are offering new attractive conditions to our customers to trade in their camera with a defective sensor and purchase a current Leica camera model.
In view of the continuing popularity of the M9 and its derivatives, largely because of the CCD technology as opposed to the current CMOS, this will disappoint many owners. However, those cameras that have already been upgraded should suffer no corrosion and will continue to perform.
As an example, you can upgrade your M9 to a Leica M10 for £4,320 (thus valuing the old M9 at a reasonable £1,430). Upgrading an M9-P to M10-P costs £4,620 (saving £1,870) and an M9 Monochrom to M10-M costs £5,040 (saving £2,210).
In addition to price details, the information sheet also discloses that the M10 is now available only in black-chrome finish.