Between adventures, roll a CON check with a +4 penalty. If you fail it, you're out 25cp in expenses. If you make it, you've created a bow of quality (effectively +1) - you can either take a 25cp loss or sell it for a 40-160cp sale price (minus 25cp in costs).

If you find exceptional trees on your travels, they could add to the sale price of your bows, make them easier to create, or even imbue a special property or additional plus.