Buyers, in my experience, normally don't make capital improvements prior to closing because of "what if you don't close." 

If rents are down while inventory prices keep going up that's the signal to sell, especially when it's got your cash flow in the negative.

Another property is a 3 family for $250k (Unheard of around here) but probably needs $200-$250k in improvements and again I fall into "Is this really going to cashflow enough or is it just a tool to fund the next one plus a small profit each year".

If they don't have a motivating factor, there probably won't be a deal there.Another thing to consider: There are ways to optimize your costs to improve your offers: Use cash or cheaper financing.

I am looking how to improve calculating ARV.

That coupled with the influx of out-of-state people moving to Miami and trying to rent as well has fueled low inventory and higher rental prices. 

Due to the low inventory it allows the conversation to be had to get a rent back for free (I have had clients do that just recently, only had a $1,500 security deposit for a $1.2M house), or renting back at the PITI for the new buyers. 

Combined with low interest rates (anything under 5%) that means affordability will further improve.

They are inexpensive and sold at most home improvement stores.  

Those cost haven't been outrageous.. maybe $1,100 over the course oy my ownership, and all of the improvement have added to the quality of the community and increased property values.