You'll be hammered for an astronomic fare if you turn up at the airport. Especially the return leg - flights are very very full - even I book closing party flights at least 3 weeks ahead ... and then (as per usual) wind up not being able to go !!!
..
I have tickets you can have if you want them as follows, but they are from Birmingham :
Out : Sunday 7th October Birmingham T1 - Ibiza dep. 06.25 arr. 10.00
[Thomas Cook - name change fee £30 - must be changed by tomorrow or forefeit]
Back -Tuesday 9th October Ibiza - Birmingham dep. 19.25 - arr. 21.05
[Ryanair - name change fee £110 - changeable up to 9th October 2pm]
Happy to assign the booking(s) gratis but obviously you'd have to pay the airline assignment fees. The outbound flight is no longer available for sale (except for a name change on an existing booking) and the return is now about £210. Birmingham airport is extremely easy if you've never been from there and I far prefer it to Gatwick. Loads of reasonable parking and even the Long Stay is walkable to the Terminal 8).