Haven’t flown UA for a year but it used to be around $22 or $25 for the whole flight with shorter duration packages available as well. Speeds were quite slow - basic web browsing type slow. You’re usually connected through the entire duration of the flight. Having the UA app on your phone helps.

I wouldn’t ever recommend paying for these types of services with miles. I think they charge some ridiculous amount like 7-8k miles - absolutely never worth it.