You may well find more excursion fares are available if you visit a travel agent to book the ticket, alternatively booking with Malev (via the agent) may give you a better fare...
Again I think AEA E377 + IB E66 is a very reasonable cost for that route. I cannot see that you will achieve much more of a saving certainly not on a scheduled carrier and as with your KLM pricing the routing/time is awful.
Beware if you find a low cost carrier alternative then if one is late the other wont refund or alter your onward ticket and you are stuck...maybe for days...or until you pay up another E200!
CSA Czech Airlines is coming up E348 BCN-PRG-BUD on the CRS for a 7 day return - that'll be a nice trip - 13:35 - 18:05 local