I am not sure how inexpernsive you are going for
The squirt gun and balsa wood gliders mentioned above are great ideas.
The klutz seiries of books are very good and there are good ones for each age you mention. I nearly always put one in my son's basket.
Maybe a small Plamobil "special" set for the youngest (pirates, knights, motorcycle riders, etc are usualy available and a hit)
Small
lego sets (the little racers in plastic boxes that run about $5-6 dollar especially) would probably still work for all of them (DS is 11 and he and his frioends still like them). They could have fun building and racing them together.
Books are always good if you have some idea of what they would like.
Silly putty, slime, silly string, etc.
If any of them wear jewely, a boy's necklace, etc can be nice and the wood bead ones are generally very cheap.
Card games (Blink, Moose in the House, Uno, Skipbo, Zues on the Lose, etc.)
Yo Yos (all ages), slinkys (maybe just the younger one), snake eggs (magnetic ovals which whirl around each other when tossed--good for all ages), etc.
Nerf style balls, darts, etc.