First of all, it is a really nice experience to have mochitsuki in your house. You need one diligent person to steam the rice, two strong ones to pound it, six to eight patient ones to form the mochi, one to wash after eating and one to clean the starch from the floors. By the way, all those persons do not have to work all the time – they can also enjoy eating and whatever social interacting.
Second of all, here is some shopping and financial info from the 30.12.2012 mochitsuki: This year we had 19 adults and 10 children. Everybody could eat enough and almost everybody got take-away packs of various sizes. One big pack went also to the family who borrowed the equipment. We had much too much of anko, so there were two big jars of it left. We used
* 8.4 kg of mochi rice
* almost 2 kg of gemmai rice
* 20 big square sheets of nori
* no kinako (somehow we forgot it, even though we had it bought!)
* half of a big daikon
* 2000 yen worth of anko (kind of handmade, around 3 litres, much too much – around 2 l were not used)
* 200 ml of soysauce
* three packs of katakuriko (each 150 g, it was just enough so can be useful to have one more pack for emergency).
We spent around 9000 yen for shopping but if we knew better how much of what is needed, it could have easily become closer to 7000 yen. If adults pay 400 yen and children 200 yen, you can end up with a more or less clear budget.