Higher levels cost more, on a kind of exponential scale that may vary by a large number of factors including the type of unit leveling, the type and number and levels of units croaked, and other activities involving the leveling unit's special abilities that may not even constitute combat. But the main thing is that you should imagine something like a logarithmic curve (not saying it literally is logarithmic), whereby low levels are achievable with a little combat, but levels above 10 or 11 become extremely difficult to obtain, and something like a level 20 unit is completely unheard of. Also notable is that not all of the numbers are available to the units and commanders to calculate, and so the existence of a predicable system of leveling at all is in the realm of the theoretical Mathamancers. Units cannot actually predict when they are going to level (Jillian is being flippant when she tells the Archons, "Cmon, it'll be fun. You'll level." but it would be a reasonable bet they might if they took out a stack of dwagons.)
This stuff needs to go into the wiki.