Have "subscription discount" sites been useful for you?

I think it depends on the site itself and your goals. The possible benefit list:

1. SEO link juice (I got some decent SEO juice from a couple sites, but no leads/sales)
2. discovery of your service for brand awareness (long term impact)
3. get early adopters who are price sensitive but can give you feedback you need

It also depends on the type of service/product you offer - something that a lot of people might need or only a niche audience. The former is better for deal sites because of a larger prospect audience size. For example, I operate BigMailer.io - a platform for email marketing, that is something that literally every business needs although some limitations exist (we require connecting to Amazon SES so extra setup steps).

The sites I signed up on cost me nothing upfront and just required I setup a unique discount code/offer for the site. One site required a membership - $29/year but I got a really good quality backlink from it, so worth it.

Whatever you do, make sure to always have proper conversion tracking setup so you can tell what works and what doesn't.

There's a lot of sites and organizations that offer to promote your product to their audience, if you will pay in cash. I find it difficult to determine if their audience is real, or engaged, or relevant for my product. The more the hide the prices, the less trustworthy they seem. My advice would be, you need exceptionally good proof of a win/win fit, because it's just too easy for a vendor in this space to take $$ and put your product in front of an "audience" that doesn't fit, isn't engaged or doesn't even exist.

Example of what you're referring to?