Its really hard to say how much its worth for a 500K email blast. How's the bounce rate though?
Perhaps you can check aweber pricing, they are well know marketing service provider, including email blast:
https://www.aweber.com/pricing.htmJust look at how much they charging and I think you will have a rough estimate as well. Plus, you will also need to provide good support, specially this niche sector.
Considering Aweber, they are charging $149 for just 25k email addresses, so if you'd want to send to 500k, that'll be 20x i.e $3000
For a recent campaign for a casino, an open rate of 8% was achieved and a CTR of 2.5% which is pretty decent when you're spamming