How do I reference the Online Price price level using SuiteScript? I want to set the Online Price level for an item based on if it's not the same as Base Price and other parameters.

same way you do in the ui, set the price level column to online price

What is the function to set a price level's value?

depends on which record and honestly which features are enabled in the account

It's the Item record itself I want to change prices for.

if you mean that actual item record, you will have to learn the Pricing Sublist / Pricing Matrix