Good morning United Airline!
I'll give you a couple of formulae to work with, because it sort of depends on a few things.
Flat rate depreciation, where Dpn = Annual rate of depreciation, C = cost of asset (list price if you will), R = residual value of the asset (i.e. scrap value) and N = useful economic life in years.
Gives the formula: Dpn = (C - R) / N
The other method (reducing balance) involves taking a certain set depreciation percentage and doing that over the written down value of the asset for each year... I suggest you see this website
which gives a worked through example.
What you may need help with is finding residual values for A320 family aircraft and for B737NG family aircraft. For this, you may need to have a look at Flight International
or another published press source... I'm not sure if you will get a reliable source of info for that here on a.net.
All the best!
It's a beautiful night to fly like a phoenix...