Still say a piece of string.
I mean if you did seo. it cost you "x"
you do ppc it cost you "y"
you do social media "z"
you do all of the above you pay "x,y,z"
marketing budget should average 10% like Nina said. (box standard)
But you must then look at industry and competition.
Look at cost per conversion which is more important.
If you have a £10 mark up and advertising costs £3 pound per unit sold then you are a happy bunny because you still make £7 profit.
If you spend £11 on marketing costs per unit then your doing some thing wrong.
Its all in the details so i'll stick with my initial comment "It cost a piece of string" but dont use shoe strings they can be counter productive.