Birgit told me that you are using the decay.txt from the zip file. That file contains only 30 data points, which by far not enough to derive the formula that you posted. If you plot y vs. x you see that this looks rather linear with a coefficient of -3 (visual estimation :-) ), so RapidMiner does not perform that bad
Does R work better on the same data set?
Do you really have 300 entries in your data? I only see 30 with a step size of 1.