The confusion stems partly from the lack of an English adjective predicate.

Using 'to be' as a transitive verb in clause where the object is an adjective both looks and sounds like a grammatical gesture in which the adjective modifies 'to be', rather than being the transitive object.

People with first languages in which 'to be' is not used in predicates where the English equivalents would include adjectives as transitive objects can easily be confused into thinking that adjectives and adverbs are somewhat interchangeable.

>"Don't pay for electricity, when you can make it simple & cheap at home."

Read literally, there is no grammatical error I can see in the sentence.

It simply isn't the statement the writer probably intended to make.

The adjectives modify 'it', which is the object of 'make', but the function is potentially confusing because of the proximity of the prepositional clause which modifies the verb 'make'. The comma is necessary in order to assign the preposition to the verb 'make', rather than to the verb 'pay'.