It burns, as per my post above (I guess we were both posting at the same time & you hadn't seen it yet).
If you lit a small fire of sticks, like a small campfire, you could burn away that purse and wallet in a matter of a few minutes. Plastic cards would melt & then burn away to ash.
Anybody nearby would smell of smoke until they changed clothes and showered.
Not only coins would survive, though - any metal buckles, press-stud closures, logos or similar would survive the fire. Metal logos if made of cheap cast alloys would be prone to melting, however.
Another easy way to dispose of a handbag and wallet would be putting the wallet in the handbag, along with a rock & throwing it into a body of water. I don't see somebody taking the time to light a fire in broad daylight, when smoke will be easily visible & burning the bag as likely, either, but I certainly could be wrong - that's just a gut feeling.
I'd be interested to know what phone Karen had, it's size & whether it's easily fit into her pockets of her clothes on the day she went missing. A large phone and or small/no pockets would suggest her phone was in her handbag & if so, I think it's safe to say her handbag made it past Blackhill Rd, as it pinged in Gisbourne, closer to Mt Macedon.