SiGe is cheaper than GaAs because it is built on Si substrate and thus
Si fabrication know-how can be applied directly on SiGe. Si is the
cheapest while GaAs is the most expensive. (4'' Si-substrate wafer ~
US$10 and 4'' GaAs ~ $340.) However, Si is the slowest and GaAs is the
fastest. Therefore, there are both Si and GaAs products targeting
different markets.
For SiGe, it is a major contender to GaAs due to its lowcost and pretty
good performance (becoming better and better due to the input of
research fundings from Si area). It may due to historical reason that
it is less common. E.g. First GaAs FET appeared in 1966 while the first
SiGe FET was in 1985.
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