Sadly, it is likely that the fuel policy will have been in the small print when you booked your car. And I know that it costs a lot less than £98 to fill up a small car in Mallorca.
I know it is a maddening rip off, but you are unlikely to get anywhere in complaining - it is just the way that that industry generally works. Any company doing a full-full rental will charge a lot more for the rental so it's swings and roundabouts, but is annoying because it is difficult to compare costs and budget, as you have found out.
I do know that all Spanish companies are required by law to have a complaints book, so you could ask to put your complaint in there when you return the car. I don't know what happens then - not very much I suspect.
In the UK, the next step would be to complain to the advertising standards authority, or the trade association for car rental companies, so if you want you could find out who these people are in Mallorca and complain to them. I suppose, having booked through Monarch, you may have some kind of contract with them, so you could always complain to them - they may do something, I don't know.
A good place for allsorts of consumer help is moneysavingexpert.com. Their article on car hire can be found here