Prices are all over the place. It depends on the model of car/truck and the style of converter. Anywhere from $20 to $900+.
https://yscatalyticrecycling.com/price-list/
A lot of scrapyards just give a set price for each converter, say $40-50 (maybe less), knowing they will lose on some and kill it on others. Ultimately, the main buyer will grade each one and determine what they will pay the scrap yard for each one. There is a chance the converter you thought was worth $500 is only worth $50. It takes somebody with a lot of time and knowledge to accurately grade a catalytic converter. Not saying all scrap yards don't have that skill, but most of them don't.
If you can get $300-500 for a converter, you are definitely at a scrap yard that knows their stuff...