The cost of service is $40 a month per line, and the monthly cost for the Apple Watch is $5. New members are currently able to sign up and get their first of month for $25 (not including the watch service plan). Once the line(s) are activated, you can continue enjoy monthly discounts by using Party Pay. I've provided some links below with more information about Party Pay.
There is a deal for the first two months running right now. $15/month
Also, instead of $40/month after the intro price, I suggest joining someone's group party so you can benefit from the $25/month/account pricing immediately! You can also choose to start your own party and pay $40/month/account until you fill the 4th spot.
As for taxes and fees, I heard in a video that they are included in the price. I have not verified this yet... my phone arrives tomorrow!