Buying extra miles for a 2014 BMW 3 or 4 Series
I drive 20k miles to work / year. Plus another 5k miles on weekends. Total 25k miles driving / year.
So, is it worth buying extra miles for a 2014 BMW 3 or 4 series lease or would be more cost effective to just buy the car?
Generally speaking, the advantages of leasing fall off rapidly once you've exceeded 12K to 15K miles per year.
The dynamics of what you're planning to do are complicated and have some sneaky side-effects. You might want to read over this discussion of the pros and cons of buying miles on your lease: