Reddit mentions of Marumiya especially pretend bottle wasabi business for 100g

Normally they are sold in either a shaker type bottle which lasts a while, or a bag with individual "sachets" as you called them, [this one has 20 sachets, 5 flavors for 1300 yen] (https://www.amazon.co.jp/%E6%B0%B8%E8%B0%B7%E5%9C%92-%E3%81%8A%E3%81%A8%E3%81%AA%E3%81%AE%E3%81%B5%E3%82%8A%E3%81%8B%E3%81%91-%E3%83%9F%E3%83%8B-%E3%81%9D%E3%81%AE2-20%E8%A2%8B%E5%85%A5-5%E5%80%8B/dp/B014883RWW/ref=sr_1_12?s=food-beverage&ie=UTF8&qid=1484847793&sr=1-12&keywords=%E3%81%B5%E3%82%8A%E3%81%8B%E3%81%91).

and to answer your questions;
yes, you can use one sachet on a bowl of rice. it is not that expensive considering each of those bags on the site you linked is about 2pound and has about 5 sachets in them, so that'd be 40p each? (idk English money tbh).

As far as how often is it is eaten, my kids eat it nearly everyday, my wife and I not so much... this is not to say that only kids eat it, but it is often used as a way to get kids a bit of veggies or what not.