Coincidently I was looking at flights to IBZ for someone else recently & something doesn't add up here.
Monarch
Sat, 21 Jul 07 (ZB 292)
London Gatwick to Ibiza
21:40 01:00 - fare £52.49
- taxes & chgs £24.28
- extras £0.00
- seats £0.00
Mon, 30 Jul 07 (ZB 293)
Ibiza to London Gatwick
01:55 03:10 - fare £42.49
- taxes & chgs £9.30
- extras £0.00
- seats £0.00
- travel insurance £0.00
So you're paying 24.28+9.30 = £33.58 in taxes and charges (Although confusingly on the payment page the breakdown says these are only taxes) [plus £3.50 Credit Card fee].
BA
Fuel surcharge rates £8.00
Insurance and security surcharge rates £2.50
ba.com booking fee rates £0.00
Taxes, fees, charges and surcharges* per person £ 42.90
So of the £42.90 you would pay £10.50 (£8.00+2.50) in charges, and 32.40 in taxes [and £3 if you pay by credit card].
So if that is the case & the taxes are the same on BA & MON then MON have just £1.18 of charges compared to the £10.50 BA want (excluding the credit card charge).
Hope this helps