Very good question!

Read more here:

• •Fees - lightning wiki
• Why is my "spendable_msat" much lower than "msatoshi_to_us"? - Bitcoin Stack Exchange
• Question regarding Commit Fee - Reddit

There is also to take in consideration:

• anchor reserve, like 330 sats/chan
• channel reserve, like 1% from total size of the channel
• commit fee is variable, depending on how much are the onchain fee at the moment.

And yes, once the channel is closed you get all back.