How much to fix my Ford Ka speedometer?

My speedometer on a V reg 1.3 Ford Ka has died. After consulting a mechanic, it seems the problem is not the display, the sensor or cable. The problem from what I'm told is a little cog deep down in the gearbox which is helpfully made of plastic and has subsequently given up the ghost. Is there an easy way to fix this, or is it really going to cost the £200-odd in labour costs for a £5 part? All suggestions or similar experiences gratefully received.