I would offer to trade marketing with the apartment complex instead of charging them. Have them put your marketing materials in their monthly newsletter, get permission to door hang their complex (with a promise to clean up the next day), and/or have them throw a promotional pizza party in their commons room or at the pool this summer.
We stuff a folded menu in every box, so our tops are left open. The space is offered for use to promote community events, charities and our cross-over marketing. We make the organization print up their own flyers, but we don’t charge them anything to put them on the box tops. I’ve had other business offer to pay to get their stuff on there, but I’ve always turned them down.
I find a 1/4 of an 8.5" x 11" size page works best. 4-up (or smaller) allows them to save on printing/copying costs and lets us fit more than one flyer on each box top when we have something we’re promoting in-house as well (like Facebook).
Right now, I"ve got flyers for a community theater, a canned-food drive and some apartment complex (with which my marketing guys are trying to get something set up… I’m hoping) alternating on my boxes. For a lot of organizations, like the local Symphony Orchestra, flyers are one of the best ways they have to promote their event.