Walmart offers the Acme Neptune Twin over Twin Bunk Bed with Storage Shelves in White/Chocolate for $349 with free shipping. That's the lowest price we could find by $150. It features a guard rail and slat support system.
Update: It's now available for in-store pickup only.
Before you spend another dollar on items and services like these, check with your public library. You will likely be surprised by what it offers for free.