If it's Glassdoor renewal season for you, here are 3 things to keep in mind:
1. The rates go up. They always go up. More traffic on Glassdoor means more audience which CAN mean more eyeballs on your page. GD owns that traffic and will charge a premium for it. It's their model.

2. Purchasing Glassdoor "just to keep competitors off our page" might be a good strategy when the investment was low but it will be hard to defend to the CFO if it comes at a YoY 30% increase. Define your metrics upfront and track them throughout the year.

3. If you do not have, track, record, look at, or report your own data, you are at the mercy of theirs. Know it upfront.

Bonus: Even w/GD, everything is negotiable. Use the "give to get" method of negotiating. Only Give more if you Get more: impressions, clicks, slots etc.