"Cost" vs. "costed" in a specific sentence, continued

Solution 1:

Like you said, costed is only used with that one sense of the word. This is backed by several dictionaries.

Oxford Advanced Learner's Dictionary:

In sense 4 costed is used for the past tense and past participle.

4. (costed, costed) [usually passive] to estimate how much money will be needed for something or the price that should be charged for something

OED (in the fine print):

5. trans. To estimate, calculate, or set the cost of. Also with out or up.
   _{In this sense the usual past tense and past participle form is costed (not cost), and (unlike in other senses) use in the passive is frequent and regular.}

The explanation you were given is mostly a red herring (as it is irrelevant)... To nitpick, he's mostly correct, but it is just a pronoun (not a dummy pronoun). It cannot be both a dummy pronoun (which is an example of a "pleonastic use") and coreferential:

Pleonastic uses are not considered referential, and so are not part of coreference.
_Wikipedia

Exception

The above is true for Standard English. In other dialects, costed may be commonly used as the past and past participle for cost, e.g Southern Mountain Dialect or inner city Sydney English.