Why don't 747s use unreliable, but cheap, jet engines instead?
The cost of a jet engine is mainly dictated by the operating conditions: high temperatures (sometimes above the melting point of the metals used) and high rotation speeds. This means you need high-performance materials even for a low-reliability engine: if you compromise material performance too much, it'll fail on the first start. 

Failures are risky. Having to shut down an engine mid-flight is one thing, but we've had some recent failures where chunks of metal were launched at high speed into the wing and fuselage. 

The industry has gone the other way: by making the engines more reliable, it's now possible to build a long-range airliner that uses only 2 engines instead of four, reducing the purchase, maintenance and fuel cost. It makes little sense to set up a special B-quality production line for a single aircraft type that's being phased out anyway. 

Failures are costly. An in-flight failure means taking the aircraft out of service to repair the failure, which means having extra aircraft on standby to replace it.