Easy dude
Look for instance in dota yo have Boots of travel, Ill use it as an example it would work like this: Make two items the boots of travel by them selves wich have the extra abilities and copy the boots of travel and add a sufix (recipe),they are basicly the same the only diference is that the recipe doesnt have any extra abilities its just a expensive piece of papper.
You also need the boots of speed to merge them with the recipe, so create them as well and give them their abilities, then the recipe triger goes like this:
Event:
Unit - A unit Acquires an item
Condition:
((Triggering unit) has an item of type Boots of Speed) Equal to True (THIS IS A BOOLEAN COMPARISSON)
((Triggering unit) has an item of type Boots of Travel Recipe) Equal to True
Action:
Hero - Drop (Item carried by (Triggering unit) of type Boots of Speed) from (Triggering unit)
Item - Remove (Last dropped item)
Hero - Drop (Item carried by (Triggering unit) of type Boots of Travel Recipe) from (Triggering unit)
Item - Remove (Last dropped item)
Item - Create Boots of Shadow the Hedgehog at (Center of (Playable map area))(OR WHERE EVER U WANT THIS IS AN EXAMPLE MERELY)
Hero - Give (Last created item) to (Triggering unit)
Special Effect - Create a special effect at (Position of (Triggering unit)) using Abilities\Spells\Human\Resurrect\ResurrectTarget.mdl
(THATS THE ONE I DECIDED TO USE YOU CAN USE ANY I THINK DOTA USES ONE OF THE EFECTS YOU GET WHEN YOU USE A TOME BUT I DUNNO WICH)
Remeber the price of the final object must be a sum of all the ingredients including the recipe if it costs anything.
If the recipe is free do the same thing, just ad lumber cost to the recipe so no one can buy it and then in the conditions exclude the part that says:
Hero - Drop (Item carried by (Triggering unit) of type WHATEVERYOUWANT Recipe) from (Triggering unit)
Item - Remove (Last dropped item)
and TADAAA! you get the item without ever buying the recipe.
I hope this helps you out!
If you still have doubts just pm me or ad me to your messenger or e mail me or what ever Ill be glad to help you out!
PD: Sorry I used Caps it was to show the diference between what was in the triger and what I was explaining.