I still hope that the foundation for the Cascade was done with possible height increases in mind. The 250' Cascade in the designs may be undersized when construction is getting ready to begin.
As for costs, the biggest factor in the costs of the projects would be personnel. The more people you have working, the higher costs will go. They may get a discount for bringing on 500 workers for the 3 projects but even then, those costs would still be higher than 5 years ago due to the construction boom that is occurring currently.
As a side note, looking at this:
http://censtats.census.gov/bldg/bldgprmt.shtml
The site tracks permits for residential construction. You can track it by County (yearly) or on a city level month and year.
It appears that in Salt Lake City through October there have been 13 building permits issued for residential buildings containing 5 or more units. These permits translate to 1,151 units. This is roughly 89 units per building.
This is up from 2014 (through October again for apples to apples) where there were 6 permits covering 242 units. Roughly 40 units per building.
This is a very large increase in just 1 year. Doubling the buildings and more than doubling the units per building.
I wonder if this will continue through 2016. Doubling the buildings again to get to 26 through October of 2016 and at least 2300 units. Doubling the units per building again though would be dramatic and put it close to 4700 units over 26 buildings. That just in SLC would lead to a large spike in population if vacancy rates continue to stay sub 7%.
It is fun to speculate how 2017 and 2018 would play out as well. With or without doubling. Just maintaining the same levels leads to big changes over the next couple of years.
It would also put SLC easily over 200,000 in the 2020 census.
I do think that this additional population will push the city to increase transit offerings sooner rather than later (S-Line double tracking/extension) and Downtown(University) streetcar lines if not also including the downtown to 9th South Streetcar line. This would be on top of additional bus services that would be needed.