I probably spent 2 hours looking for a decent small-sized cross-body purse under $50 before deciding that I could have spend that time making my own. Over the years I've collected a number of purse parts including cute frames and chains, this clear bow frame I got on etsy. It's actually pretty easy to make your own frame purse by hand without a sewing machine, especially in felt which is very forgiving.
Making the pattern is the hardest part, but by digitizing it and then laser cutting it, I ensured that both the pieces were identical, would fit perfectly into the frame, and the cut-out pattern was just for grins. You could just as easily make a pattern on paper and cut it out of paper, however I use felt because I don't have to bind the edges. Felt won't fray.

The yellow felt is extra thick, bought online at TheFeltPeople.com (fitting name, huh) , the gray is just generic stuff from any old hobby store. I prefer acrylic felt for laser cutting because the edges melt slightly and don't turn black or stink like wool felt.

Step 1: Tracing the Frame

Tracing the frame is the first part, because of course, you need to know how big the top of your purse should be. It's pretty easy, really, just hold it down and trace with a pen. Make sure you get the contour where the hinge part is so you can mark on your pattern how far to sew up the sides.

Step 2: Draw the Rest of the Purse

The rest of the purse is up to you. I wanted this one to be able to fit dollar bills on the bottom, but you can make it as deep as you like, you can add box pleats, or just make it flat.
I decided to add some small gussets just to give it a little extra pouf.

Step 3: Digitizing the Pattern

This part is up to you. If you have access to a laser cutter, you know how nice it is to be able to cut the exact same thing over and over again without having to lift a pair of scissors. If you don't have a cutter, you can try Outfab for a fun selection of felt and good prices. They also have some thick felt in stock.
I prefer hand-drawing the pattern just as a hands-on experience, I usually get good ideas when I scribble around, but technically I could have gone with a ruler to the frame and drawn up my pattern directly on the computer because now that I have a rough draft of the pattern, I use a ruler to measure the frame and ensure I am within a milimeter of the desired size. I also of course use geometric shapes to ensure the pattern is perfect.

The decorative cutouts are just little flower shapes I copied and pasted. Next you can cut out your pieces, two for the inside in one color and two for the outside in another color. I use CorelDraw for graphic design.

Step 4: Assembling the Cut Pieces

I cut two pieces in yellow for the inside layer and two in gray for the outside detail, the cut-out detail showing the yellow felt underneath.

To ensure they don't wiggle around as I sew the gussets I dab some fabric glue onto the gray and press it down on the yellow.

Then I get to work on the gussets, hand sewing them up until they are connected. You can see how the gussets added a little shape to the bag, poofing it out so more things can fit into the purse without it looking swollen.

Once you get the gussets sewn, sew the front and back together up the sides, but just below where the hinge goes on the frame.

Now you're ready to attach the frame (it's so easy!)

Step 5: Attaching the Frame

This part is deceivingly easy. Some frames have holes so you can sew them to the purse bag. Some materials are more difficult to work with but I think felt is the easiest. Stitches hide themselves, glue soaks in, it's super forgiving.
The important part is to squeeze the glue into the frame, do NOT try to glue the edge of the fabric and shove it in, that will be a sticky mess.

I use good ol' E6000 with a narrow tip (surprisingly hard to find, I buy mine by the dozen whenever I can find it) I wouldn't use craft glue or fabric glue for this part because you need it to be super sticky and hold together while it dries. I fill the frame part up about 1/3 with the glue and then slowly shove the edge of the purse into the frame. You can use a flat-head screw driver to help with this part. Make sure you've got the frame with the hinges going the right way or you'll again end up with a sticky mess from having to undo it.

Double check inside and outside of the frame, close it, open it, shove the edges in again...you do not want them to slip out!
Once you're satisfied the edges are shoved into the frame as much as they will go, let the purse sit for a few hours to completely dry.

Then you can add the chain. Don't go trying to add the chain before the glue dries or the purse will probably slip out of the frame (sticky mess!)

Step 6: Adding the Strap

Lots of purse frames have attached two little metal tabs with holes for adding a strap. You can make a removable strap with some small split rings and some chain (any hardware store or purse findings site will have these things)
You can use snap-clips if you like for easier on/off of the chain but I like the continuous look of split rings.