Hi Laura, you are right. Don't be spending loads as pup could be destructive. I have a second hand cot mattress since puppy hood with a removable washable cover. Inexpensive and works perfectly.
Rogz has beds that they claim to be indestrictable, water-resistant and have removable and washable covers. So far my 15 week old pup has not torn even the smallest hole on it, so maybe they are right
We use vetbed... completely indistructable, washable, dog has slept soundly on it from when he was at his breeders and he's now almost 9 months old.