Not sure if there are car boots or auctions in your area, when we lived in Playa Flamenca there were weekly auctions in a bar run by Brits and Irish, gas bottles were regularly sold there, and empty ones made around 10 to 15€, this was because if you go to a garage with one and swap it for a full one there are no questions asked.

you would have to pay me an awful lot more than 45 euros to go to the effort of taking my gas bottles down to some nasty brit bar with Guinness on tap or perch myself at the local market amidst the made in china brigade. If the OP wants to pass by and pick one up on the way to a day trip to Gib he/she is welcome to collect

How do you know that it is a nasty bar with Guiness on tap?

artistic licence

Thanks for your replies everyone, I've sorted what I am going to do!

don't forget your toothbrush