Below is a sample search storage estimate for environment with 500GB content databases. This was calculated by adding up the sum of MDF files. There are so many considerations for storage, and this is just a rough sketch of the storage aspects related to the Search Service Application. This does not consider base installation overhead nor does it factor in other SharePoint Service Applications (SSA), Office integrations, User Profiles, Usage Database, PowerPivot, etc..
ContentDBSum = Measure the size on disk of MDF files. For this example, 500 GB.
IndexTotalIndexSize = ContentDBSum * .035 or 17.5 GB
QueryComponentIndexSize = TotalIndexSize / Number of Index Partitions
StorageQueryComponentIndexSize * 3 single query component with space for index merging
QueryComponentIndexSize * 4 single query component with space for index repartioning or 70 GB
TotalPropertyDBSize = ContentDBSum *.015 or 8 GB
TotalPropertyDBLogSize = ContentDBSum *.0031 or 2 GB
TempDBSize = TempDBSizeContentDBSum * .00034 or 1 GB
TotalCrawlDBSize = ContentDBSum *.046 or 23 GB
TotalCrawlDBLogSize = ContentDBSum *.011 or 6 GB
ContentDBSum *.011 = TempDBSize = 6 GB
SearchAdminDBSize = # items in index * .3 or override as 5 GB
In this case, I don't know how many items there will be in the index so I am going to allocate 5GB.
Backup Size = basically, add all of the database sizes together.
Microsoft Technet (2010). Performance and capacity test results and recommendations. Retrieved April 27, 2010 from http://technet.microsoft.com/en-us/library/ff608068(office.14).aspx.