I recently priced out my cats on Pets Best and they would insure one of my older girls (14) who has only ever had issues with arthritis at $31 per month plus $22 PER month for the routine maintenance coverage. They would not cover Smokey, I don't know if it was the diabetes or the pancreatis/liver issues or the combination of it all. They would only give me the $10/month accident coverage on Smokey. That coverage was still with a $250 deduct for 80% coverage on up to $7000 per incident, $100,000 lifetime coverage. That's still a lot of money out of pocket at the time something happens. I figured it'd be better off putting that money into a savings account each month, then it is there to cover whatever goes wrong, no exclusions.
Since the diabetes is pre-existing, no costs associated with that will be covered if anyone does cover you, I didn't see any that covered any pre-existing conditions. Bloodwork is often considered routine, so that may not be covered unless a maintenance package was bought.