Does anyone have rule of thumb cost estimates for this kind of work?

This not only lets your get a discount when these suppliers compete against each other, but if you have detailed plans from an architect or a spec for the build out - they can also estimate all the materials needed for you.

Conservatively estimating new construction costs as $300/sq foot, the new duplexes could cost about $600,000 each to build (plus interest costs).

Photos will not work on estimating.

Basing estimates of rents and expenses based on my existing knowledge of local markets.

I can’t get my budget in order because this one builder won’t give me an estimate until I’m ready to apply for permits.

(vii) would seem to indicate that you'd be acting as an unlicensed broker.(1) "Broker":(A) means a person who, in exchange for a commission or other valuable consideration or with the expectation of receiving a commission or other valuable consideration, performs for another person one of the following acts:(i) sells, exchanges, purchases, or leases real estate;(ii) offers to sell, exchange, purchase, or lease real estate;(iii) negotiates or attempts to negotiate the listing, sale, exchange, purchase, or lease of real estate;(iv) lists or offers, attempts, or agrees to list real estate for sale, lease, or exchange;(v) auctions or offers, attempts, or agrees to auction real estate;(vi) deals in options on real estate, including a lease to purchase or buying, selling, or offering to buy or sell options on real estate;(vii) aids or offers or attempts to aid in locating or obtaining real estate for purchase or lease;(viii) procures or assists in procuring a prospect to effect the sale, exchange, or lease of real estate;(ix) procures or assists in procuring property to effect the sale, exchange, or lease of real estate;(x) controls the acceptance or deposit of rent from a resident of a single-family residential real property unit;(xi) provides a written analysis, opinion, or conclusion relating to the estimated price of real property if the analysis, opinion, or conclusion:(a) is not referred to as an appraisal;(b) is provided in the ordinary course of the person's business; and(c) is related to the actual or potential management, acquisition, disposition, or encumbrance of an interest in real property; or(xii) advises or offers advice to an owner of real estate concerning the negotiation or completion of a short sale; and

I now track my final sales cost against my original estimated ARV against what the best/closest comp was when I first did my ARVs. 

@Rahul Sivaswamy if you use the BP Rental Calculator, it defaults to 5% Vacancy, 10% PM, 10% Maintenance, etc.These are all pretty good estimates, except the Vacancy factor of 5%.

Deduct NEW property taxes after you buyDeduct home insurance costsDeduct maintenance percentage, typically 10%Deduct vacancy+tenant nonperformance percentage(we recommend 5% for Class A, 10% Class B, 20% Class C, good luck with Class D)Deduct whatever dollar/percentage of cashflow you wantNow, what you have left over is the amount for debt service.Enter it into a mortgage calculator, with current interest rate for an investment property, to determine your maximum mortgage amount.Divide the mortgage amount by either 75% or 80%, depending on the required down payment percentage - this is your tentative price to offer.If the property needs repairs, you'll want to deduct 110%-120% of the estimated repairs from this amount.Be sure to also research the ARV and make sure it's 10-20% higher than your tentative purchase price.As long as the ARV checks out, this is the purchase price to offer.It is probably significantly below the asking price.