Fantastic!
Third class is about £7.50, second class isn't much more.
First class is probably not that much either but I didn't look.
That's if you book direct onto a Ukranian train, not sure if you can book online or need to buy at station. (I'm not using these, I was looking it to it at one time).
https://ua.tutu.travel/poezda/rasp_d.php?nnst1=2218000&nnst2=2200000&date=17.05.2018&isRedirect=1I'm getting train from Warsaw and couldn't book online on the above (well, if you can I couldn't work it out). You might be able to book online as it's not international.
If you can't book online on the above, and you don't want to take chances paying at station, you can book via these but more expensive (but still very cheap compared to our own trains - they also charged for delivery after the online price)
https://booking.polrail.com/