Have you contacted local churches for rental purposes? This can be a great option.
Dollar store for some of the decorations...
Some folks host pot luck style showers to cut costs... If that's not your thing than try to see who can contribute <like family!> and collaborate your own food spread. Catering is far too expensive, it's much cheaper go your own route with as much help/contributions as possible. Good spring frugal foods: fresh fruit salad, large leafy green/raw veggie salad, cold Italian pasta salad... and an easy entree like assorted mini sandwiches or homemade flat bread pizzas cut into individual squares. Both can be made to cater to vegetarians and meat eaters. Dessert as you wish.
What's the head count?
"Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime." ~Chinese Proverb~