Favor gained = .ceil((character level/15) * creature level). Symbol use favor cost is level dependent. It is relatively more expensive to both advance and use symbols at low levels. A level 15 character killing a level 15 undead gains 15 favor ((15/15) * 15) with a symbol of return cost of 188 favor. That equals approximately 12.53 same level kills per use. A level 100 character gains 667 favor per SLK [.ceil((100/15) * 100). The sym of return cost for a capped character is 2174 units which is ~3.26 SLKs (2174/667) per use.
https://gswiki.play.net/Favor
Mark