If I just went with a regular direct vent 50 gallon tank the price would be much, much cheaper. However, because the quote I got for a 75 gallon direct vented high BTU tank was within $350 of the tankless, and because of the fact that 75 gallon was a very borderline fit in the cramped space I was considering, I decided to go with tankless. The guy claimed his dealer cost for the tank was super high. I initially thought he was pulling my leg so I went online only to find that the online direct sales cost was even higher than his claimed dealer cost, so he was telling the truth.
As for the hard water, I have a whole house carbon filtration system, which definitely helps.
Mind you instead of sticking with the tankless quote he gave me, I told him to install an electric buffer tank as well. I'm getting a 6 gallon electric tank heater placed after the tankless. Since the water coming into it is usually already heated, it should be inexpensive to use. This solves the problem of the added tankless delay, and also solves the
cold water sandwich issue. It also solves the tankless minimum flow rate problem. Tankless units won't activate unless a certain threshold flow rate is reached, usually 0.4 - 0.5 gallons per minute.
In terms of condo usage though, it's all moot as a retrofit. Tankless isn't a drop-in replacement. The high BTU requirement and specialized ventilation requirement changes everything. Plus they're higher maintenance, which is something condo dwellers generally may not like. In my condo too, there was no water filtration system or water softener, nor was there room for one in my unit, which means if I didn't religiously maintain the tankless, it'd probably go on the fritz within a couple of years.
Tankless can work in condos though if the condos are built to support tankless. They, and tanks for that matter, are less suited for high rise condos, but for condo townhouses for example, tankless does make sense. The building designs would have to include proper gas pipe sizing, proper utility room locations for inexpensive but appropriate venting options, and properly placed drains.