Oh, another way to go, which would be effectively zero cost, would be to see if the Principal would allow the children to be "Principal for the day". What that would actually mean would be something that would need to be defined by you and the Principal, but the idea of "being the Principal", and maybe being awarded with a certificate, would mean a lot to the kids and could be used for all grades (one a day).
I would recommend trying to find a prize that the kids would really desire. For example, WebKinz were/are very popular. What we did last year is use a WebKins that is the same as our mascot, the Husky. The kids loved it!
My youngest suggests markers and a notebook; my older child suggests a batch of home-baked chocolate chip cookies.
You might be able to put together some gift bags fairly economically if you go to the Dollar Store and receive donated items, but that's a lot of gift bags.
Maybe you have some overstock SpiritWear, like school T-shirts or tote bags.
Books are always good prizes; and if you conduct Scholastic Book Fairs, they tend to donate a percentage of sales worth of books.
Or, if your PTO holds Movie Nights, Christmas Shop, Winter Carnival or such, how about free admission for a family of four? Or a free pass for a year to all PTO events?
If your PTO holds raffles, maybe $15 worth of raffle tickets.
-or various combos of the above
Maybe some movie passes or a gift card to a book store? We have done those in the past and they were very popular. You could just do a $10 or $15 gift card and the kids would be happy. We have also been able to get some movie passes donated by a local theatre. Even if you only got a few, that's a few less that you need to buy.