What is a cost-efficient, fast and easy method of transferring btc between own computers?
[Edit: Read comment as to why this is not a good idea.]

You could just store the wallet.dat file on both computers, which contains your entire wallet. Then both computers will be able to spend from the same wallet.

Do make sure to copy the wallet to the other computer from time to time: if you don't synchronize regularly, the balances might start mismatching because the private keys are no longer the same.