Yes a metal frame would definitely be a lot safer in terms of fire hazards. You're smart and I doubt i need to explain why.

With regards to pricing, the standard retail price for wood frames are much cheaper. What you can do is to DIY and build your own metal mining rig frame. There are lots of people out there who have used metal or aluminum shelves instead of making a custom mining rig frame. It will depend on what is available at your local hardware. You can always look for online guides out there on how to make mining rig shelf. Good luck.