You can get old console WISE terminals
and special cards that will allow up to
16 serial connections for the PC... however,
you wouldn't get X on those, just console.
I've seen some of those dumb terminals
advertised for 40 bucks :)
If you want X you'll need to get thin-clients
(graphical terminals) which are a bit of
a different pricerange, and for performance
reasons use Ethernet rather than serial :)
Look at this thing
and its "bigger brothers"