What the fee usually buys
A paid listing usually buys speed, a higher position for a few days, or a link that search engines follow. The directory's pricing page says which.
Speed matters when you have a launch date. Position matters only if the directory has real visitors, which you can check by looking at how many votes or comments its recent listings have.
Start with the free queue
Submit to the free queue everywhere first. After a month, look at your analytics and see which directories sent visitors. Those are the ones where a paid slot might pay for itself.
Fees we don't pay
StartupSeen uses free listings only. If you want a paid slot, you buy it from the directory yourself, so the listing and the receipt are in your name.