Backing/caching ration

Hi all,

I'm thinking about buying an SSD, I would like to
know if someone can, more or less, suggest what
could be a reasonable size, in relationship with
the backing device.

At the moment, I've 1TB RAID-10, of which /usr is
24GiB (10GiB used), /home is 64GiB, and then there
are two "work area" volumes of 250GiB.

Some other space is spread around /var and similar,
and there is some backup space, for future use.

Any idea on what could be "minimum" SSD size (due to
cost, of course).

Thanks,

bye,

-- 

piergiorgio