I'll start with the pay part - I wouldn't pay anything for a review. I would provide a free copy in exchange for a review, but to actually pay wouldn't feel right to me. Though since this is just Tapas and it's already available for free then I definitely wouldn't be paying anything. When I self-published my short story, I paid for a promotion from a website that got me way more than enough purchases to cover the cost of the promotion, but for my ARC reviews, the only thing offered was a free copy of the story in exchange for a review.
I would pay for beta reading/editing/etc and I do pay my beta reader. Based on your other post, it feels more like a beta reading service you're attempting to offer, but to be a beta reader, you don't publicly post the issues with a novel. It's a service for the writer to see where they can improve before what they've written starts getting real reviews.
When it comes to a promotion that you're offering - how many people are signed up for your newsletter? What are you site traffic stats? How many unique visitors do you get in a week/month/etc? These are all things I would be looking at before reaching out and paying for a promotion.
For the reviews - I would try to get your own set up going in how you review things. That way people can see and know what to expect if they request a review from you instead of focusing on what the writer wants you focus on, focus on what sticks out to you. Have sections to rate each part - cover, summery, story
If it was beta reading service, like mentioned before, that's why you listen to what the writer wants you to focus on. Some beta readers want just over arching story cohesiveness where as others want something more in depth.
It really sounds to me like you should offer a review section and a beta reading section, but that's just how the post/wording comes across to me! 