Is there any study or calculation on how much money I save if I buy door/window X with a certain R value vs door/window Y with a different R value?
It's a very straightforward calculation, though parts of the industry prefer to pretend otherwise. Also, math is involved, and that tends to scare off people with math scars from school. Dumbed down approaches give dumbed down, generalized answers.

First, establish if you are dealing with Metric or "English" (BTU based) R values. They are similar but the units are different. Since it seems likley that you are dealing in square ft and BTUs and that's what I do normally as well, I'll go that way. Also note that some items (windows, quite commonly) will be specified in U values, which are 1/R. Let a spreadsheet do the math.

Then add up the areas of your house that are various constructions at various R- values (which you'll have to figure out from looking at how they are constructed, if you don't know) - ie, 1200 square feet of attic at R50, 2400 feet of wall area at R-12 (or 2400 square feet - 42 square feet of doors and -100 square feet of windows) and 42 square feet of doors at R-4 or R10  or whatever you are comparing and 100 square feet of windows at r1 or R3 or r-5 or whatever you are comparing. Beware of "as-built .vs. insulation-only" R-values for things like walls, where having a wooden stud at R1-per inch every 16-24 inches really adds up over the whole house. Or, ignore the parts of the house OTHER than those you are considering options for, as those won't change until you change them. 

1200 square feet of attic at R50 will require 1200/50 = 24 BTU per hour per degree F to heat. 

2400-142 square feet of R12 wall will require 2258/12 = 188 BTU per hour per degree F to heat. 

42 square feet of R2 door will require 21 BTU per hour per degree F to heat, while 42 square feet of R10 door will require 4.2 BTU/hr/F 

100 square feet of R1 window will require 100 BTU/Hr/F and R10 window will take 10, while R3 window will take 33.3

Now, look up the annual heating degree days for your area, and you have some number (say 6000) of days * degrees for a typical heating season. Multiply THAT by 24 hours per day and you know the seasonal load for each part of your house - 6000*24*100 = 14,400,000 BTUs for 100 square feet of R1 windows, 4,800,000 for R3 windows. 

Now follow that down the "how you get your heat and what that costs" path (btu per unit of fuel, efficency of how that's converted to heat delivered to the house) to sort out the annual operating cost difference for the options you are considering. Then consider the lifetime of your windows, and likely trends in fuel costs. For electric resistance heat, 100 square feet of R1 window in a 6000 HDD climate is 4,219.2 kWh/year; r3 is 1/3 that, R10 is 1/10 that.

For a more complete heat use modeling job you should also consider ventilation rate, but again, not that much of a factor if you are only looking at "this window .vs. that window." There are also models that attempt to account for solar gain from windows, but that is difficult to model well in a complex environment with shading, etc.