It is almost always considerably cheaper to use existing moulds than to redesign and make new ones. There may be unusual exceptions to this - perhaps if the original mould was badly conceived.
New moulds need to produce many castings to pay themselves off. Thats why you pay a lot for creatures/dragons - they weren't sure about the fan base so they covered the mould costs by charging more per unit. Unfortunately, the higher price can also reduce the volume of sales (which appears to have happened for dragons and creatures given the response seen on this and other websites.)