1. Who is building the majority of high-rises? Is it larger investment firms, or individual developers?
See above answer, add Westbank, Wall Financial, Aquillini, Concert, Omni, etc... take a look at the UDI BC site for almost all the companies involved look in the link section.
Almost all of them are privately owned, a few are public but none of them are large investment firms. Office Towers are a completely different ballpark and are almost exclusively built these days by pension fund money.
2. What is the average floorplate size of a downtown high-rise?
5500-8000sqft with the majority around 6000sqft.
3. How much are developers buying these sites for in and around downtown?
Varies way to much to give you an answer you could use, it all depends on density potential, zoning (residential allowed?), how long ago the property was purchased etc... Best recent purchase to give you an idea would be to see what Millenium paid for the SEFC site. That'll give you a clue how much it costs.
4. What is the profit margin for these developers? And is that margin on total sales
The larger the risk the larger the profit margin will have to be. Some people believe they make 20-25%. Truth is if a developer can walk away with 10% they're happy. You wouldn't see so many developers fail if they were raking in the money.