So I use the community uncapper. I cap all my skills at 300. I think, but I'm not sure of it, that any points I gain in speech past 100 are useless as far as reducing prices is concerned. I'd like to make it so that for each point of speech (even those past 100) has an effect on the prices, but also adjust how much each individual increase in speech effects the reduction so that at level 300 the net reduction is the same as at 100 in vanilla.

Sorry, if that's not clear. I basically just want to spread it out so speechcraft growth so it's slower and converges to vanilla at level 300.

Anyone know how I can do this in the CK?

You can do that with the uncapper - it is in the uncapper ini - this section:

[SkillExpGainMults]; Set the Skill Experience Gained Multiplier to; betwen 0.0 and 10000.0; Using too low or too high values may crash the game. Handle them with caution.; Default game value = http://forums.bethsoft.com/topic/1384858-how-could-you-make-speechcraft-scale-more-slowly/1.0

then something like this will slow the leveling down. when I last ran the game I opted for 1/3 the leveling speed - like so:

fSpeech=0.3333

Thanks for the info. However, I don't think that would fix the issue with the levels over 100 not giving any benefit for prices. See the formula is:
price factor = (fBarterMax * (100 - skill) + fBarterMin * skill)/100

So once skill reaches 100, the maximum benefit is achieved. I wonder if there's some way to change that 100 in the formula to a 300. I'm not even sure where that formula is stored or if it's accessible....

I don't know - because to me unless there is a reason to go that high - why do it?

You could keep the scaling of other skills which do have benefits to go that high, but then gimp this so at least your not a talking god that quick.