What is the cost of the engine Boeing 747 Model 400?
The actual prices paid for an engine are likely to not be released publicly. However, list prices are advertised, and other sources may help give an idea of the price range for such engines. The 747-400 model has three engine options available: PW 4056, GE CF6, and RR RB211. These engines are also used on other aircraft, which will help in finding the prices.

The CF6 engine has a list price of about $11 million (from an A330 order in 2009, higher thrust version; \$12.2 million in 2015 USD1).

The PW4056 engine has a price of $5.8 million (from a 1994 report; \$9.3 million in 2015 USD).

I haven't found good pricing for the RB211. An older, lower thrust version was priced at £340,000 in 1971 (\$4.8 million in 2015 USD2), or you could buy one right now for $35,000 (the seller hopes it can be put on display, mentioning "very high end art displays" but also "top dollar in scrap").

The list price for a 747-400 was about \$240 million, which means that 4 engines at a list price of \$10 million each would be about %17 of the cost of the plane.

1Inflation calculator
2exchange rate