Bg Lots here has a LOT of earbuds for around $5, incl the brands that are $10-$15 at Walmart and other stores. I found really cute erasers at Dollar Tree - esp a 2 pack of ones that look like the arrow keys or the delete, home, end keys of the computer keyboard, those giant novelty pens/pencils, a necklace of blinking xmas lights (better early gift so they can enjoy it for a few days), etc...
Think back to when they were little. Did they have a favorite card game, like pickup sticks, etc....? I have often found that those are far cheaper now than they were back then, plus they bring back fun memories.
In our family books are always a great gift - but they have to be opened LAST or nothing else gets opened because the person is reading, lol!
Consider a fun mug, esp the newer ones that look like paper coffee cups (look for ones made of plastic and not ceramic so they don't break as easy), or the reusable plastic cups with straws that look like fast food cups.
Do they have a favorite college or sports team? Look for things with that logo.
Think about gift certs for an outing with mom or dad. To a movie, for ice cream, to a bookstore, whatever they enjoy doing.
Check for classes at local craft/hobby stores and community centers/arts centers. A certificate for a class or set of classes is a great gift.
Look for a higher end version of the axe type products. It can be a way to get them away from the super strong smelling products an can be a nice indulgence also. Bath and Body Works has a CO BIgelow line that is nice and has several choices in the men's products. If they have a favorite scent consider getting the essential oil or a container of the scent and some unscented shampoo, lotion, etc.... and scenting it yourself (affordable option when some of the prices of those things are super high - plus you have some control over how strong the scent is and of the quality of the base product - esp important if there are allergies. We do this because so many of the products make eczema and my migraines act up).
What were their favorite treats when they were little? A brand of chocolate or cookie could bring back fun memories.
Consider giving a note telling them of some favorite memories of the 2 of you, or telling them how proud you are of them and who they are growing up to be. Include ONLY good stuff, no "but when you XXXX" after you say the positive thing. This can mean a lot, esp to a difficult child.