Please give more information:

Which exact trains would you like to book? Date and departure times would be helpful.

What price did ÖBB quote?

@rvdborgt 

We would be traveling 17/6-18/6

Interrail website:

Ljubljana-Sevnica (Slovenia) 9.05pm (free)

Sevnica (slovenia) -Split (Croatia) 11.30pm (14e seat, 34e with bed)

ooeb: only beds available and price around 76e, same route as above

The price is 74€ per couchette (Sevnica - Split), also bookable via the ÖBB website (using these steps). There seems to be only 1 seat left.

Last year, this was also a quite expensive train.

Without logging in, Interrail's reservation module quotes some reservation fees but they are incorrect. If you log in, it says "unavailable” indeed. Looks like a bug to me. @Nanja, someone needs to look at this.

There may also be a separate night train Zagreb - Split (leaving around midnight), like last year, but I don't see it in the planners yet. That train would be cheaper. In addition, Croatian railways only start selling 2 months in advance.

@aino 

The Nighttrain of ÖBB/ZSSK (Austrian/Slovakian Rail) is quite expensive on this specific train you get usually just few %discount on the standard fares as it´s a seasonal train and for ÖBB/ZSSK quite expensive to operate this train :/

Usually the Nighttrain Zagreb- Split is the best option for you but as @rvdborgt  said currently couldn´t be found in the Railplanners :/

This train will run only 07/01/2022 to 08/27/2022; it’s not running at weekends (sunday night); it should not be possible to book it from outside Croatia. The timetable for this train can be found even at the RailPlanner app. But it’s not running 06/17/2022 anyway. 

For the ZSSK train the price shown at oebb.at is the correct price for this train; 14 € or 34 € for this train is totally incorrect (correct prices: 54 € seat, 74 € couchette (6), 84 € couchette (4), 94 € sleeper (t3), 114 € sleeper (double), 164 € sleeper (single)). 

Best wishes, Hektor.