Link Unavailability is caused by atmospheric factors related to weather. The typical behavior is few very short (few seconds) to short (tens of seconds) signal attenuations or signal drops. Those issues culminate with storms, rain and snow – for more details see ITU recommendations (ITU-R P.530 and related).
Higher Availability (with constant Distance and Capacity) is always more costly (both CAPEX + OPEX). Typical relationships are expressed in the following chart:
Typically, an increase of Availability from three to four nines (from 99.9% to 99.99%) is not very costly, while increasing to five nines (to 99.999%) is 1.5x – 2x more costly and then increases exponentially.
In practice, an increase in Availability by one nine (for example from 99.99% to 99.999%) requires an increase of system gain by 10 dBm (to increase fade margin). Each additional +10dBm can be realized by a number of technical solutions (up to the limits of physics):
Each technical or license improvement means additional cost (CAPEX or OPEX or both).