I'm confused about the gold value of filled soul gems. The UESP says the value of a filled gem is the value of the soul multiplied by the value of the unfilled gem, but in-game it seems like they're actually added together. With BTB's mod, petty gems are worth 5 gold. A mudcrab soul value is 5, and the gold value of a petty gem with a mudcrab soul in it is 10, not 25. A lesser gem with a nixhound is worth 30.
Is the UESP wrong, or am I missing something?