Sunflower seeds. I love em, they are maybe $2.50 a pound raw. Some of my favorite healthy snacks are
A banana, drizzle with 1/2 TBS agave nectar (a bottle is about $5 and lasts me about a month), sprinkle a bit of cinnamon on top and 1 tbs sunflower seeds
Sweet potato, nuked in the microwave, sprinkled with cinnamon and agave nectar
Soup- I make soup 1x a week and freeze it in small individual-sized containers. A butternut squash or carrot soup with some lentils or black beans and corn is one of my favorites, it is low cal, about 100 cal for a cup, but fills me up and is super quick to heat up in the microwave
Hard-boiled egg or a fried egg on steamed greens (spinach, kale or brussels sprouts)
good luck! Wow, that is a lot of restrictions. I do like the blog www.wholelifenutrition.net/ which does have lots of recipe ideas for gluten-free.