By far the easiest and cheapest 'recipe' (found it a while ago on another thread on here):
Cut crust off bread, spead with marmite, roll up, then nuk in microwave for 1min30sec or so. Turns out nice & hard & easy to hold.
Bits don't come off very easily, it's nice and soft, my boy loved them. And soooo cheap and easy! You can also just do them without marmite and they're still good (just squish them together well).
There's also this one here
linky
but I found my boy wasn't keen on the texture, and bits broke off.
And this one that someone on here posted once that I copied to try! Haven't tried it yet though.
Muesli teething Biscuits (exactly like rusks, but a heck of a lot cheaper)
1C flour
1C Farex Muesli
1tsp Baking Powder
2Tbsp Spreadable butter or Margarine
1/4C Sugar
1/2C prepared formula
Mix everything except milk in a bowl. Rub butter through until completely mixed.
Add milk in a think stream and mix. Stop adding milk when you can press the dough together to make a ball that is not dry and crumbly. Roll into an evenly shaped cylinder 20cm long. Dip a sharp serrated knife into cold water and cut into 24 pieces. Flatten or roll into crayon shapes (I did this, easier to hold).
Bake at 150 degrees celcius for 20-30 minutes until lightly browned. Leave to cool completely before putting into container.