What's more common is to spend the money on printed copies which you send to a number of reviewers with a letter asking them to look through it. Even if it's an ebook, you send a paperback, which means there can be a relatively big cost to it compared to other activities.
I'd never pay for a review at any cost but for those that are considering it ... the expense is high - I think Kirkus is like $450. Printed copies with postage is about $6.50 for a 320 page book printed with Createspace that's 70 reviews (assuming you make sure that you have a high likelihood of a review as opposed to just blindly sending).
In my whole career the only "marketing" budget I ever allocated for is review copies -it has an extremely high ROI, so I wouldn't classify it as "relatively big cost compared to other activities" - just my opinion of course.