Just up the price of your final products to reflect the more expensive materials.
If you spend 200% on nicer materials and then charge 3x as much for the final product, you can sell half as many and make a better profit margin with significantly less time.
The market is saturated with inexpensive coasters, maybe it’s time to leverage the custom angle instead of joining the race to the bottom on price?
Inexpensive cork ikea coasters:
Slate:
Hardwood maple:
Or if you want to make and sell lots of less expensive sets, go even cheaper, with paper:
Bonus there is that you could laser up a stamp and use the stamp to “letterpress” the paper coasters.
Anyway there are a million approaches to these things. I’m sure you’ll find a process that works for you.