I have purchased several perfectly serviceable bass clarinets for less than $500. Cheapest was an Evette (Malerne) hard rubber for $200.
I bought an M. Lacroix wood for $327. Then I bought a Conn hard rubber for $500. The most I ever spent on a bass was $1,050 for a brilliant extended bell, low-C Moennig bros wood from the 1930s. The pads were about that old, but they all sealed and worked perfectly.
I never had to do any work on any of those, and I sold them all for a profit, except for the Moenning which I still retain.
This was all about 10 years ago, so I'm sure things have changed, but when you look at every new listing in the clarinet category on eBay, every single day for over 10 years, the deals find you.