My senior prom I asked a good guy friend of mine to go. My mom and grandma went in halves on my dress ($250 for an off the rack bridesmaid dress as we could not find a gown in my size at Macy's or Penny's or store of that sort back in 1998). My date/friend paid for his ticket, my wrist coursage, and for dinner. I believe I paid for picutres and his boutiner. Plus, he drove us to dinner, to prom, after prom, and the back home. Collectively, I'd guess we spent no more than $500 total. Our prom tix were $50, dinner was probably another $50-60, pictures was around $30, my dress was $250, not sure of his tux though. Proms have gotten soooooooo expensive. It's sad.
Anywho! I'd take some of the suggestions others have replied with here (take them to a nice park/uni/hotel and do "nice" pictures that you can the upload and print whatever sizes you want for cheaper than a pic package would be).