Rail pass for Central and South-Eastern Europe rarely makes sense, since traditionally ticket fares in almost all the countries of the region aren't very high, but let's look in details:
Prague to Poprad: daytime EuroCity or couple overnight train options, tickets for EC trains (seats, daytime) start from only 20 euro, for the overnighters cheapest couchettes are 27 euro, sleepers - 32 euro. However all of the above are discounted tickets offered in limited number per departure, thus purchase in advance is required in case you don't want to overpay. Place to buy it -
Czech Railways (CD) ticket eShop. In case you don't want or can't buy your tickets in advance, then full standard fare for route Prague-Poprad-Tatry when purchased in Czech Republic is 39,9 euro
Poprad to Bratislava: easy and relatively cheap. Trains go like every hour during the day, standard full fare when purchased online via
Slovak Railways (ZSSK) website is 14,17 euro. At the ticket counter it will be a bit more expensive, since online purchase gives additional 5% discount. Also you can take a bus, but it will take an hour or even two longer compared to the journey by train
Bratislava to Budapest: again easy and nonproblematic. During the day trains run every two hours, full price is 17,5 euro no matter when you buy your ticket as far as it is done in Slovakia (or Hungary). Also possible to travel cheaper via Sturov/Esztergom (good town to spend couple hours), but it will extend your total travel time
Budapest to Bucharest: here you have a choice of either daytime train or one of the two overnighters, in both cases journey is quite long (14-16 hours). Hungarian Railways sell discounted tickets for all three direct trains - seats start from 29 euro, couchettes from 39 or sleepers from 59 euro. But again discounted tickets require in advance purchase, since they are limited per departure and can get sold out. We have a step by step guide on how to purchase tickets via Hungarian Railways (MAV) website -
http://europetrainsguide.com/Advice/How-to-Buy-Train-Tickets/Hungary-MAV.htmlBucharest to Sofia: here things get a bit more complicated, since train tickets (roughly 25 euro for one-way ticket) can't be purchased online + quality of carriages might be not that good. However you still have daytime or overnight option for the trip
Sofia to Thessaloniki: Greece canceled all the international trains, so at the moment rail option isn't really available, but there are plenty of direct buses from Sofia to Thessaloniki (some of them actually run by Greek Railways company). One-way bus ticket (depending on departure) cost 40-50 BGN (app. 20-25 euro)
Thus, as you can see yourself, it is not very difficult to cover the whole itinerary in less than 100 euro in total...doubt that any pass would beat this price.