I'm searching for the cheapest SSR transponder and transciver with
8.33 channel separation that could be mounted on a sailplane.

Mode S transponders cost in the range of about $2500 to $2700. There are
several manufactorers, all more or less in the same price range
(Garrecht, MicroAir, Funkwerk, Becker...). You need a level 2, class 1
transponder, be sure not to confuse level and class!

As far as I know, there's no need for a glider to have a 8.33kHz radio
anywhere in Europe. 25kHz radios cost in the range of around $1300 to $1700.

Of course prices may vary and depend on exchange rates.