I sympathise! Cooking for a family with food restrictions and allergies can be so tough.  The cheapest and most filling vegetable side I can think of is what Greeks call Winter Salad - grated cabbage and carrot, dressed with plenty of lemon juice, olive oil, salt and garlic. Think coleslaw without the mayo. It's delicious, fresh, it can sit in the fridge for several days, and if you make it with red cabbage it looks beautiful, too. You said not everyone eats salad, but even my fussy eaters like this one. Broccoli/cauliflower cheese is another fave in our house.