Actually, you can share an app. I do this with certain apps I want all my children to have on their iThings.
On your iThing go to Settings > Store > Apple ID (all the way at the bottom) and change it. Then go to iTunes or App Store and download/install the app you want on that iThing just as you would on your iThing (be sure you do it on yours first). Then after the install is complete, you simply go back to Settings > Store > Apple ID and change it back to the iThing ownwer's info.
Now when there is an update for that/those apps it will ask for the password of the app owner ID not the iThing owner password, but really that's no big deal. It takes about three seconds to type in your password when your child yells across the house that they need your password and you yell back "Bring it to me!" (Oh, that's my child and in my house, sorry).
P.S. I find that if it's a small purchase app, it's easier (and better for the devs too) to go ahead and pay for it multiple times. Most apps are just $.99 and seriously if I've saved to own or give an iThing then I think I should be able to buy the app too.
(edited to correct most typos )