The cost of leasing an aircraft is based on the price (and that is determined by what is included in the price - some customers will try and add non-capital components for tax and/or liquidity reasons), cost of funds, customer risk profile, use profile (projected cycles & hours), term (length of lease), projected market/value at end of lease, and tax.
In the good old days, leasing companies and financiers would place 'block' orders, first to get a good price (partly for their bottom line and partly to pass on to customers) and second to influence the market (leasing companies would often sell delivery positions at a profit).
In 2005 the market is very different, and leasing companies are somewhat pre-occupied with protecting US airline exposures, and assisting with the re-structure of that industry.
Leasing business arises in different ways in 2005. Larger airlines negotiate aircraft purchases direct with the manufacturer, and then in parallel (or later if delivery is well into the future) arrange funding. Some will use negotiating expertise from a financier or leasing company to assist in this process. Later, the airline decides whether to lease or buy the airframes, engines, simulators and other equipment.
Smaller, less financial airlines may use the buying power of the financier or leasing company to acquire the aircraft for them, either to lease or purchase.
If an airline orders 10 aircraft, they may lease 5, purchase 5, yet purchase or lease all the engines. It's simply a case of the best deal, with the best overall cost.
Tax can play a big part. There are sometimes advantages loading early aircraft deliveries with high levels of non-capex costs, some airlines operate their own lease companies (often based overseas), others lease overseas, while others may appear to borrow overseas. Big capital items - big mirrors.
So leasing companies are really driven by customers. No customers wanting to lease - no lease company orders.
Finally two points. All current 787 orders and most A38 orders to-date are what financiers would call options. They are supported only by miniscule payments to the manufacturers, which are refundable, transferrable & deferrable, so in virtually all cases, finance is yet to be arranged, although negotiations are well underway for the A38 (care not to double count previously announced airline orders and later lease company orders for the same aircraft).
Second, the A38 is regarded as a medium-high risk (size for A, some technology, project magnitude) and 787 is high risk (technology, manufacturing, degree of risk-sharing) projects, and so financiers and leasors have very much preferred to take a wait and see approach.