We have no modding tools for easy finding certain item, so we need to search for each item through the game file's text. At first you should use some good text editor such as Notepad++ or EmEditor. Then open 'catalog.sp' file, press Ctrl+F and search by keywords.

To change ammo limits  for each ammo type
<item key="AMMO_REVOLVER
[scroll beneath]
<multiplicity>                      here are ammo quantities         
<item>           
<quantity value="40" />          start quantity limit           
<slotid>SLOTID_ANY</slotid>         
</item>         
<item>           
<quantity value="10" />            bonus from upgrade 1           
<slotid>0xE655E53D</slotid>         
</item>         
<item>           
<quantity value="10"/>          bonus from upgrade 2     

Change weapon prices.
Open catalog.sp and search for certain weapon line. Example: <itemkey="WEAPON_REVOLVER_CATTLEMAN   
Same for each weapon like  <itemkey="WEAPON_RIFLE  or  <itemkey="WEAPON_SHOTGUN
Scroll down and you will see its price in cents - that's what you need to change. 
Use notepad++ or EmEditor for work.


All items names that you need, usually begin with <item key="
For horse breeds it will be <item key="HORSE_BREED_     
Example:
<item key="HORSE_BREED_MUSTANG_BUCKSKIN">

Prices will be below, in cents.  
<item>CURRENCY_CASH</item>               
<quantity value="10000       - it means 100$


Animal carcasses:
PROVISION_ANIMAL_CARCASS_
PROVISION_SKINNED_CARCASS_
For beaver it will be:
PROVISION_ANIMAL_CARCASS_BEAVER_
PROVISION_SKINNED_CARCASS_BEAVER_

For pelts  there are names 'hide',  'fur', 'pelt', 'wool'.
Examples:
<item key="PROVISION_RAM_HIDE_POOR">
<item key="PROVISION_RAM_HIDE">
<item key="PROVISION_RAM_HIDE_PRISTINE">
... or
PROVISION_BEAR_FUR_PRISTINE
PROVISION_BEAR_LEGENDARY_FUR_PRISTINE
...or
<item key="PROVISION_RABBIT_PELT">
or
PROVISION_SHEEP_WOOL

MEDICINES:
CONSUMABLE_MEDICINE
CONSUMABLE_MEDICINE_USED     =opened
CONSUMABLE_POTENT_MEDICINE
CONSUMABLE_SPECIAL_MEDICINE
CONSUMABLE_SPECIAL_MEDICINE_CRAFTED
.
CONSUMABLE_RESTORATIVE     -  =bitters, same as above
.
CONSUMABLE_TONIC
.
CONSUMABLE_HORSE_MEDICINE
CONSUMABLE_HORSE_STIMULANT
.
CONSUMABLE_HAIR_TONIC
CONSUMABLE_COCAINE_CHEWING_GUM

HERBS:
item key="CONSUMABLE_HERB_GINSENG
item key="CONSUMABLE_HERB_ALASKAN_GINSENG
item key="CONSUMABLE_HERB_AMERICAN_GINSENG
and so on



How to change the effects of Opened medicines restore 20-30% instead of 100% ?
In the top side of the file:

<item>     
<key>EFFECT_HEALTH_10</key>        --example for  CONSUMABLE_MEDICINE.     
<id>0xC7BCE4CF</id>          
<value value="2"/>                               ---2=20%; 10=100%. This effect is for all CONSUMABLE_MEDICINE except golden cores.
<percent value="100.00000000"/>     
<time value="0"/>     
<timeunits value="0"/>     
<durationcategory>0x5C26356D</durationcategory>   
</item>

I mean, the one common percent for ALL non-golden health tonics. This is how the game works. And the same way for deadeye and stamina tonics.