brand new aircraft cost hundreds of millions just for one
For widebodies perhaps, current delivered prices for A320/737-800 around $45m and for A319/737-700 around $38m.
Leases will kill you because you will have more expensive airframes plus a bank that wants to make a healthy margin on it too!
Rental factors for new aircraft are around 0.82%/month giving a rental for a new A320/737-800 of approximately $370,000 per month and, for the A319/737-700, approximately $312,000 per month. So, very much more expensive than used 737-300s and, unless you can maintain very high year-round utilisation, difficult to justify a switch into new equipment. Hence, I guess, why used 737-800s being added to the fleet. However, the "bargain of the month" must be used A319s which are available at very low lease rates - even for relatively young aircraft - and I would have thought would make the perfect 737-300 replacement in due course even allowing for type difference).