There are City specifics that you'll either need to (A) learn or (B) pay someone to know for you, but much of the cost and complexity and time comes from the CA-wide plumbing, fire, mechanical, and other codes. 

But it's an art, just like choosing a good doctor or auto mechanic. 

No problems with previous landlord.Income:  $2000/mo plus $1200 tips (he is a waiter and has been employed at the same restaurant for 3 years)Savings: $9,500Credit score: 685 all accounts current, late payments several years back on a credit card that is now paid offDebt: Credit card and auto loan payments: $255Nick and Nora - couple moving in together, both were living with relatives and have no rental history.Income: $5800/month - he is a mechanic, she is a call-center employee both have stable employment historySavings:  $150/$50Credit scores: 695/455; Betty has collections and multiple late recent paymentsDebt: Car payments and credit card debt $1600/moTom: just finished graduate school, looking to start a new life, stable housing history, looking for a full-time job in his masters discipline, working at 2 part-time white collar jobs he has held through school.Income: $3100Savings: $1200Credit score: 775, lots of debt, but pays on time. 

I prefer mechanically cleaning the drains,  especially if you want environmentally friendly. 

Fiverr for SEO is like hiring the fiat mechanic down the street to service your Ferarri... they'll be able to work on it... but they'll probably break something or permanently damage something in the process. 

The fees for every transaction + the mechanics of going through the custodian for everything is a pain.

To name a few:Wall modification and drywall patching for new ductwork locationsFloor plan modification - You will need to build-out of mechanical closets to house the furnaces for each unit which will alter the floor planA/C - Not sure what the existing cooling system is, but the new heating system could effect the cooling systemElectrical - May need new electrical panels to accommodate the furnace load, & wiring to power the new furnace unitsWith that said, I think this could easily cost over $5,500 a unit when you consider these additional scopes of work.I would highly recommend getting in touch with a local HVAC contractor who can provide some insight on the construction considerations & costs as apart of your due diligence.

So do whatever let's you sleep at night.But you really asked about the mechanics of paying extra.  

So don't know the accounting mechanism the finance guys used. 

So, apple can the like can layoff as much as they want, it can help the rest of this country's industry with their modernization / mechanization / softwareization / whatever...So, I think from that point of view we are saying the same thing.As for your last point, I think again I would say the same thing, perhaps differently.