Calculating a monster's 'score'

Okay. So again it depends how specific you want to be.

If all you want to do is incorporate defense, you can just use EXP/HP * (100 - DRate). Ideally you would compute this separately for physical and magical damage, but to combine them you can do this: EXP/HP * (200 - PDRate - MDRate)

 Spoiler

I used every 20th mob to make a quick sample, higher is better:

Monster	EXP/HP * (200 - PDRate - MDRate)
Snail	36.0
Potted Sprout	28.8
Flyeye	24.6
Toy Clown	24.4
Dark Axe Stump	19.9
Mateon	18.7
Curse Eye	16.8
Cold Eye	16.1
Wild Boar (PC)	15.0
Patrol Robot S	13.2
Master Chronos	12.0
Mummydog	11.1
Black Ratz from Another Dimension	6.9
Crimson Balrog	4.1
Cronos	9.6
Fire Sentinel	9.8
Transformed Snack Bar	13.7
Play Seal	8.8
Poison Golem Level 3	5.6
Trixter	8.5
Jr. Lucida	7.8
Pink Teddy	7.5
Bloctopus	7.1
Lazy Buffy	0.0
Dark Drake	5.8
Gold Captain	7.0
Krip	6.0
Barnard Gray	6.1
Emergency Guard Robot L	5.7
Guard Robot	5.5
Tick-Tock	5.3
Giant Snowman	6.7
Octobunny	43.3
Racaroni	4.7
Athena Pierce's Clone	3.0
Griffey	12.5
Dark Sand Dwarf	4.1
Security System	4.0
Firebrand	3.9
Sage Cat	3.8
Gigantic Viking	3.7
Master Guardian	0.2
Chaos Blue Wyvern	1.0
Gatekeeper Nex	3.1
Cold Shark	3.5
Skelegon 1 in Cave	3.4
Yeti	3.3
Freezer	1.5
Maverick Type D	3.4
Coolie Zombie	3.3
Krexel	1.2
Lilynouch	2.8
Chaos Horntail's Legs	0.6
Witch Cat	15.0

Then you can multiply this by mobs/area to give a crude overall measure.

If you're allowed to use data from your own damage range, you can calculate expected hits per kill, and from there approximate time per kill (factoring in mobs/area; either more hits or fewer mobs means more time) and then calculate exp/time.