Correct use of "hereby"

Hereby: As a result of this document or utterance:

If you want to use hereby, the sentence might be:

The total amount specified in "Appendix 3 Price Breakdown and Payment Plan" is hereby attached to the Subcontract.

This sentence now means, 'As a result of some discussion prior to this sentence being committed to paper (assumably immediately before this sentence or at least referenced somewhere in the preceding document) the total amount which is formulaically specified in Appendix 3 is calculated and the number attached to the sub-contract.'

edit: To retain the meaning of your initial sentence without any of this implication, using the as you indicated, without hereby, is the way to go.

"Hereby" is not an adjective, so that's one reason that your sentence is strange, but there's another, more important one.

"Hereby" is part of a linguistic category called "speech acts" or "illocutionary acts." When you say "hereby" you're saying that, by your very utterance, something is true.

For instance, if the President says "I hereby declare today to be National Cheese Celebration Day," it's National Cheese Day simply by his say-so. If I say "I hereby forfeit my right to an attorney," my saying so constitutes the forfeiture.

"Hereby" is hardly the only one; perhaps a familiar example of such a speech act is "I now pronounce you husband and wife."

Both usages are incorrect since hereby is an adverb but you are attempting to use it as an adjective.

Also that is not a sentence as it lacks a verb. It is a subject (The total amount) with a long prepositional phrase (specified in..) with a nested prepositional phrase (attached to...).

I think you are looking for something closer to 'this', 'enclosed', or 'attached' depending on the context.

"The total amount is specified in Appendix 3, Price Breakdown and Payment Plan, found in the attached Sub-Contract."