The period between 2010 and 2050 is broken down into nine five year periods in the Calculator. We use one cost for each of these periods.
This cost is generally calculated by linearly interpolating between the 2010 and 2050 figures - that is to say for the middle period, 2030 to 2035, the cost will be half way be between the 2010 and 2050 costs; for the fith period the cost is five ninths of the way between the 2010 and the 2050 costs.
In the case of the power sector we have followed the way the costs fall in the MARKAL model. The costs do not change in every period from this source, with typically three or so "vintages" of costs. For example one cost for the 2020s, another for the 2030s and then a thired for the period after 2045.
The actual numbers for the costs and the calculations involved can be found in the excel spreadsheet version of the model, although the 2010 and 2050 costs displayed on the wiki give a good approximation of the trend.