If you have deflation, this is a gain (p is negative).
Now, you are right that this doesn't entirely compensate the "gain" due to inflation, of the increase of R.
Indeed, you can say that R_t2 = R_t1 * (1 + (t2 - t1) * p). So R_t2 is bigger (in nominal value) and it "brings you" indeed an extra amount of cash which is (t2 - t1) * p * R_t1.
If your sales price were exactly equal to your cost, that is R_t1 = W_t1, and you would not have created value, then the COST of inflation, W_t1 * p * (t2 - t1) would cancel out ENTIRELY the 'benefit of inflation' R_t1 * p * (t2 - t1), because it is equal.
In the same way for negative p (deflation) of course.
However, if you create value, that is, R_t1 > W_t1, then you do have a small difference: you gain somewhat if there is inflation (on your benefit) and you loose somewhat if there is deflation (on your benefit)... at least in nominal numbers. However, as your gain in nominal numbers, can now buy LESS, that is even corrected for a second time.
You still don't see that it is not the same for deflation. In deflation you may end up with less money than you started with, i.e. R_t2 < W_t1 due to prices falling, even though, on paper you may easily get R_t2 > W_t1 * (1-(t2-t1)*i). But the latter won't make up for the losses, which will be real. If you write for deflation what you wrote for inflation, you will see the absurdity of your reasoning. Why did you not do this?
As I said before, you can indeed write something like R_t2 - W_t1*(1+(t2-t1)*i), but this won't be your profit, since it is a premium that you get or lose over another investment which gives you i%. If you get losses due to deflation, i.e. end up with less money in your pockets, will you be better off than if you hadn't invested your money at all?