score:0
The cheapest option might well be plane and bus. I'm seeing prices as low as 46€ one-way from Barcelona to Málaga, and then 13€ for a bus from Málaga to Gibraltar.
For the flight it's worth checking a comparison website: the 46€ price was for a Ryanair flight, but Vueling are also in the region of 50€ on some flights. Prices obviously vary according to time of day and how far in advance you book.
For the bus there's no choice: Avanza is the only option, and there's no price advantage to booking in advance, although there is the benefit of being sure you won't arrive and find that the bus is full.
Note: Málaga is one of the largest airports in Spain, so you should allow plenty of time for the connection.
Upvote:1
You can get the train from Barcelona to Algeciras and then either get a bus or taxi. There are more details in the page of the man in Seat 61 here. Note that section starts in London ut since it includes an overnight in Barcelona it should be easy enough to pick up the route. Spanish trains are not that expensive but your opinion might vary from mine.