If you take 2-3 extra cards it will hardly be noticable on your packing. Cost is also fairly insignificant, as long as you avoid the largest capacity/fastest cards.
Then if you find your usage is ridiculous (lots of high res video perhaps) you should be able to get more cards whilst on holiday, as long as you have a few days warning...
There are so many factors involved that we couldn't come up with a fair estimate of how much space you'll use.
Camera resolution, RAW / Jpeg or both, number of shots/video all effect things drastically.
Even 'shooting like crazy' gives a very wide band for the number of shots some would feel averaging 200/day fits that description yet at some events I've taken well over 2000 in a day. (I think I may have reached 4000).
If you don't take spares, start using your smaller card. If you fill that in under half your holiday & have to switch to the other you MIGHT run out, and could use lower resultion/more compression (or review & delete) if things look awkward.