The transaction costs are variable depending on the speed with which you want your transaction processed. Miners have an incentive to include transactions with the highest fees in the next block, so when the network is congested with tons of unprocessed transactions, you'll be paying more.

And that's only looking at on-chain mechanics. Solutions exist to remedy issues with scalability of the underlying blockchain.

Interesting. Does it mean Bitcoin is not immune to DOS attacks where basically me and my friend transfer 0.0000001 btc between each other repeatedly for giggles.

My understanding is that the miners would have no incentive to include the transaction, and so each transfer would take a long time to be processed.

To, if you don't mind waiting until the lull of the day you only have to pay like 30 cents.