In the encyclopedia section of this site, there is a link to see all the units on a spreadsheet. Look at the differences there.
Maxed out 4* might have a total power of 30000-60000 thousand once you add up their land sea and air attack. A 5* have 75000-115000... some a bit less, and some more. And then 6* 140000-288000, with an exception of a 116000 named Charles, but he has an S speed and shreds things up.
Best thing you can do is Max out 4*, sell them for Time Elixers, save up for a 5*. Then do the same with 5* to get 6*
As for how much, you can find the cheapest ones for about 35 TE right now. Youll see Fiona around for 15-20 Time Elixers, but he starts are only as good as a really good 4*, and you cwn get her for inviting 10 friends to play the game and they use your invite code.
Hope all this helps and good luck!