R-Value is a measurement ot resistance to conductive heat transfer / loss. It is the mathematical inverse of U-Value, the measurement of actual conductive heat transfer, or loss (If you take 1 divided by the R-Value, you get the U-Value). The U-value shows the proportion of heat allowed to conduct through.
When we consider the U-Value, we get some rather interesting information (Well, interesting to an energy efficiency junky like myself). For example, take the 2 R-values discussed, R-30 & R-50. R-30's U-value is U-.033. R-50's U-value is U-.020. This means that R-30 stops 96.6% of conductive heatloss & R-50 stops 98% of conductive heatloss. A switch from R-30 to R-50 gives a net gain of only 1.3%. Unless you live in an extreme climate, it probably would not be cost effective to add the difference, unless you have an accessable attic & can blow more insulation on top (retro blowing in an open attic is realatively inexpensive. Retro adding a layer of foam & re-doing the ceiling is not.). Also, if you have an R-50 attic & double the insulation, the 2nd R-50 also stops 98% of the heatloss, but only 98% of the remaining 2%.
You will notice that the focus of R-value is on conductive heat transfer (the main heat transfer mechanisms are conduction, convection, radiation & evaporation/condensation). For many, this is the primary focus of energy efforts. However, conductive heat transfer is not the most important in homes. The most important is convection, the heat tranfered by air movement (you walk into a cold room. you notice 2 things, the walls are not insulated & the window is open. What do you solve first? Shut the window!). In fact, to accurately measure R-value, or conductive heat transfer, convection must be elliminated.
Why do I bring this up? Simply put, If you have 2 apparently identical houses with radically different energy costs, the reason is not minor fifferences in R-values. Rather, the reason is one house is tighter than the other, thereby controlling convection better.