In D&D 3.5 and its younger sibling/child/sleek and deadly archnemesis cloned from a piece of its DNA (Pathfinder), there is a table you can look at if you're trying to make a "balanced" monster. If I wake up from a particularly worthwhile nightmare and decide to lob its monstrous protagonist, tentacles writhing and hellmaws grinning, at my unsuspecting players, I can look at that table to figure out what reasonable stats might be for my homebrew beastie to challenge a party of any level. The table is especially useful if I don't know the rules for advancing monsters, templates, assigning DCs to supernatural abilities, etc... or if I just don't want to take the time to build a rules-legal monster from scratch.
What if I had a table like that to compare PC power levels? It would let me compare my players' numbers to see how they stack up to other builds. To me, player numbers are much more important to balance than monster numbers: I can flub, adjust, and flagrantly lie about what happens on my side of the screen, but I want to adjust loot and challenges so that the PCs are within the same strike zone of fun and challenge, so that I can merrily fling my nightmare beasts and baseball metaphors at them and know that the entire party will be equally and appropriately challenged by them.
I was going to sit down and start working on that table... until the thought struck me: a monster of a given CR should be an even match for a PC of equal level. The chart should work for PCs as well as monsters, if all you're trying to do is gauge where on the power curve they fall.
Let's try a few examples (PCs written without magical gear):
Level 2 dual-kukri ranger, 18 strength. Attacks at +5/+5, deals 15 average damage. Significantly higher than the +4 attack, 7-10 damage CR 2 monster.
Level 8 dual-kukri ranger, 20 strength. Attacks at +12/+12/+7/+7, deals 41 average damage (using very sloppy crit math). The CR 8 monster attacks at +15 and does 26-35 damage if all its attacks hit. This lines up well with the table: “a creature with lower than normal attack bonuses will often deal higher damage”.
Level 2 enchanter, 18 int. Enchantment spell DC 16. Destroys its monstrous counterpart, whose primary ability DC is 13. By level 8, the enchanter is throwing DC 22 spells while the monster is still at 18.
This project will take some time and doing- the first place to start will be to look at the “Pathfinder Iconic” characters and see how they match up. I'm betting they will match the creature design table very closely... and that the actual PCs I've seen at my table over the past two years will look nothing like those numbers at all.