For Christmas, it's...
About $20-40 for important people (parents, sisters, best friend since age 8, boyfriend, his family). Often combine parental/family presents--allows me to get better stuff, not junk. :)
Under $10 for advisor, planned to give something to dept staff last year but it was a food-related idea and I got sick during the week I was going to execute it, so no go.
Homemade treats for grad school friends. I'm famous for my oatmeal-raisin cookies. :D
I look for cute cards on mega-sale after Christmas and use them the next year. :)
For birthdays it's actually similar, but I don't (typically) give my parents, my bf's family, or department/work people birthday gifts. For friends/coworkers I either make treats or buy them a drink.
Don't have kids, but growing up we didn't get much for Christmas (actually New Year's, but the traditions stem from the same place); typically $10-50 in gifts per person depending on year. For our birthdays it varied widely; often this was a time for my parents to get us big-ticket items we needed anyway, it was just nominally attached to the birthday. One interesting factor, my parents rarely got us toys or games (most of those were from friends). We did get sports equipment, craft tools (new markers!), books... We were also raised not to give money or gift cards. As teenagers, our parents stopped picking out gifts on our (overseas) grandparents' behalf and relegated that task to us, but as younger kids we didn't ever get money or gift cards from family.