This idea that 737s are cheaper doesn't add up. To give you an analogy, in the UK the capital costs of BMW 530 are lower than a Vauxhaul/Opel Omega because of lower depreication Similarly it is cheaper to lease a new BMW 530 than an old one because of the structure of discounts. If you try to lease 150 seats from GE then there is no reason to suppose they will charge you less for an old aircraft versus a new one.
And to support this what do all the operators do as soon as they can? - order brand spanking new 737NGs