The codebox can actually reference the variable resultItem to get the properties of the item returned, so you could use resultItem.price to get the gold value of the base item. I think that value will be different than the actual value of the actual item given if you're using random affixes that end up changing the price of the result, though.
This probably won't entirely work as a replacement for the synthesis plugin, but you could use my recipe list extension in tandem with the default crafting plugin. Recipe list makes it so that the player can raft known recipes from a list, without having to put the individual ingredients into the slots every time, or having to keep mental notes of the ingredients involved in each recipe. You could set it up so that the player learns recipes through experimentation, and then can synthesis them more easily from the recipe list menu instead. Of course, if you used that, you'd also need to modify it to make use of the fifth ingredient slot you've added, so that might add some difficulty to it.
As for your custom synthesis things, I don't have a particularly good replacement for those. You can use an <on creation eval> note tag from item core on the items, and do all kinds of modifications to the result item if it was given to the player on the crafting scene, like attach augments, or use item upgrades on them, but without knowing exactly what you were doing with synthesis, suggestions for replacements will have to be vague like that.
You could also look into the 'Additive Ingredient Traits' I have in this plugin to add some stuff to your crafted gear from the ingredients used. Those aren't random, though.
I am aware of that plugin by DreamX, but I've never used it myself, and I'm not entirely sure there's a way to get it to work together with my system. I'd need to look more closely at how it works to see how it does what it does, and see if that can be taken into account.
-Ramza