A race to the bottom on pricing is just going to get you very low quality customers which in turn will tank your sending reputation. I think you're approaching this backwards.

In my experience everyone appreciates lower prices - and good senders should be rewarded with lower prices.

If you allow spammers on your network things get a lot more expensive. So you can't offer low prices and allow low-quality senders. That's why we have a strict AUP and aggressive measures to detect spammers, phishers, and non-permission (aka cold-outreach) messages.