It depends on the agreement with the Rugby club as far as stadium rent is concerned. It could be that the agreement is for each meeting and a double header might cost more for rental (I obviously don't know, which is why I say "could").
There will be two lots of points money, two lots of away travel money, four sets of tyres and perhaps extra medical cover expenses too. The tyre cost alone will be in the region of £1200.
As a recent guide, Somerset recently charged £23 for the double header with Newcastle, whilst the return double header at Brough was £25.