The output transformer (should!) be the most expensive component in an amplifier. Not only does it have a significant effect on the quality of the tone of an amp, but also the reliability.
The components used in a decent transformer cost more than those in a pickup, and transformers are far complicated to wind than pickups, so I find it quite odd that so many people seem to object to paying more money for amplifiers, whereas they are prepared to pay for expensive pickups.