How much does it cost to lease a jetliner?
Slide 19 of this IBA presentation (PDF) shows how the aircraft's age is a factor:



Other factors such as global outlook, supply and demand, fuel prices, etc., all have an effect.

Anyway, that presentation confirms the accuracy of the data on an airliners.net thread from 10 months ago, from which:

For a 737-800 you are looking at USD 120,000–350,000 monthly.
For a 747-8i you are looking at USD 640,000–1,250,000 monthly.