A royalty free sound effect means that you pay for the right to use one specific audio file or a subset of similar files. The terms can vary from a yearly subscription or in perpetuity. It depends on how much you pay and the nature of use.
There are advantages in purchasing a royalty free sound effect for your video. For one thing, you could have a higher quality sound than what you find at the free sites. It depends on the vendor. The site might have various audio lengths that meet your needs or a wider selection of effects.
If your video is being used for commercial purposes it might be safer to use a royalty free service.
BeatSuite.com does have a specific type of sound effects for sale. I'd describe them as futuristic, steampunky or functional. I could imagine that these effects could find their way into an audio drama or podcasts. The standard license covers use for Internet use, web videos and podcasts.
The quality is very good and the price of the tracks reflect the quality. Start at $50 and work your way up. You can sample the tracks before purchasing. You might want to review their guide to royalty-free music so that you are clear on their approach and definitions.
The Music Bakery has sound effects for sale starting a $1.98. I just tried out their squeaky door and it gave me the shivers. The audio files are available in mp3, wav and aiff use for you MacUsers.
It is your call if you want to do that kind of thing. The point of royalty free is to be able to use it they way you want without attribution.Be sure to read the web site's legal notice as well as the terms of service.
RoyaltyFreeMusic.com has sound effect albums for purchase. They also have individual effect audio for sale but this is where it gets tricky.
You can buy an individual track for around $5 or get a whole album for about $100. Sounds about right.
For personal and non-profit use they are free with required attribution of where you got the sound. For commercial use you must purchase a license. The free sound effects are short and are in the mp3 audio format.
These web sites are easy to use, you can review the tracks before you purchase and they make clear what you need to understand before you buy.
I'm not a musician or sound effect expert. I do know with the
growing evolution of media consumption that there are hybrid products
like books that will have videos embedded in them or textbooks with
practical exercises with beeps and clacks a plenty.
If you think that you might need a creaking door or a metallic zapper you might want to check out these or other vendors.