Well, as it turns out, there are hundreds of items that a photographer will very likely think is the cats pajamas on Christmas morning that are under $100. Spare lens caps (front and/or rear) - we're always misplacing those things, speedlite diffusers, filters, microfiber lens cloths, memory cards, spare batteries (dedicated, AA's, and rechargeables), camera bags, reflectors, grey cards, GC for the ol' print lab, mats, frames, digital frames, animoto subscriptions, magazine subscriptions, etc. etc. etc. You get the idea.
If your photographer is jumping into off camera flash techniques or needs equipment for the pro or ad-hoc studio, I've compiled a list of some great light modifiers and related equipment all retailing for between $4 and $80, not including shipping.
PBL Photo Studio 40" Reflective Umbrella Softbox by PBL. This is a set of TWO umbrella softboxes for only $29 smackers. These work more like softboxes but setup and break down more like umbrellas. The advantage over umbrellas is confining the side light spill and saving it to send all the light where you want it.
Big white seamless. If your studio is not blessed with a cyc wall, you are going to want a big white seamless backdrop. Great for people but also versatile for product work.
3. Lens Pen. Whether it's just dust, or the fingerprints of the kids you just photographed, you need to clean your lenses regularly. This thing is only $4!
Cheap flash triggers. Just a godsend! I have some of these and they are terrific. You need to know that they do not communicate TTL info and only synch to 1/250 and start to peter out around 50 ft. But they work (with speedlites, but also with old focals, starblitz, etc.), the manual nature will really up your game, and they are so cheap you can leave spares around your studio so your buddy can pick a transmitter and shoot that model with you.
5. Wind machine. At $78 this is the priciest thing on the list, and honestly, you can probably pick up a shop fan at Walmart for far cheaper. But this is fancy lookin', ain't it?
6. Softbox with grid and flashmount. Pretty much explains itself. The grid is really what makes it.
7. Impact Umbrella Bracket. Your photographer is gonna want three of these. Trust me. I believe Impact is B&H's house brand, so pretty reliable stuff.
8. Cheap shoot-through umbrella. If you don't go with any of the softboxes above, you can pick these up cheap. If your photographer doesn't have wireless flash triggers, I've found that the shoot-through is handier for aiming the infrared triggers over the reflective umbrellas. You lose and spill more light with the shoot-throughs, though.
9. Another softbox with bracket and case. Another good option for size and price.
10. Lightstands. Lightstands to stick under all the brackets, triggers, umbrellas, and softboxes above. This is a set of two for under $40.
If I can pull it together I'll try to do another gift guide soon for some of that other stuff I mentioned (waaaay up there in the ol' post). Never stop pressing that shutter!