What you need to do is go and pick a standard frame that would fit your 5 photos. Say at Kohls, Wal Mart, KMart etc. Make sure its big enough for the 5 of them. Then go to a framing store (preferably a small shop not the big stores) and have them cut you a mat to fit your five photos that will fit into your frame. You can lay them out in whatever order you like and they can make the mat fit the pix. Also you can consider cropping off just a little if possible to make them fit into the frame.
If you lay out your photos.....l to r....5x7 horizontal....then 5x7 vertical (drop the vertical pic down a bit to center between the four horizontal ones)....5x7 horizontal you can leave 1 1/2 inches on the outer edge and an inch in between the pics.
Your top and bottom edge will be bigger but that can be remedied by adding a little hole for pic info below each pic.....so vertically you would have mat, pic, hole, mat, pic, mat, hole, mat...this needs to be evened out to look nice. Its hard to do without the pics in hand. If you want to keep cost down tell them to leave the space with out making an additional cut in the mat and you can add the info your self by printing on paper from the computer or writing on the matt if you have a steady hand.
Hope that makes sense. You can probably easily fit it into a 20x24 frame. Plus if you buy it at a discount store you will have your glass already and backing board. Saves money.
Guess what I do for a living....
At our store it cost 2.50 per hole with the first being free with the mat plus the cost of the mat Should be around 20 bucks give or take a dollar with the five holes. Still all in all this is the least expensive way to go.
Hope this helps